Fix potential crash in SQL Server column retrieval code

Before this commit, scaffolding could crash with an `ArgumentNullException`.

```
System.ArgumentNullException: Value cannot be null. (Parameter 'column')
   at Microsoft.EntityFrameworkCore.Utilities.Check.NotNull[T](T value, String parameterName) in Microsoft.EntityFrameworkCore.Design.dll:token 0x6000111+0x1a
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.GetPropertyName(DatabaseColumn column) in Microsoft.EntityFrameworkCore.Design.dll:token 0x6000221+0x0
   at System.Linq.Enumerable.SelectListIterator`2.ToArray() in System.Linq.dll:token 0x60001dc+0x20
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source) in System.Linq.dll:token 0x600011a+0x1b
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.VisitIndex(EntityTypeBuilder builder, DatabaseIndex index) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600022d+0xa2
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.VisitIndexes(EntityTypeBuilder builder, ICollection`1 indexes) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600022c+0x28
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.VisitTable(ModelBuilder modelBuilder, DatabaseTable table) in Microsoft.EntityFrameworkCore.Design.dll:token 0x6000226+0xea
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.VisitTables(ModelBuilder modelBuilder, ICollection`1 tables) in Microsoft.EntityFrameworkCore.Design.dll:token 0x6000225+0x28
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.VisitDatabaseModel(ModelBuilder modelBuilder, DatabaseModel databaseModel) in Microsoft.EntityFrameworkCore.Design.dll:token 0x6000222+0x78
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.RelationalScaffoldingModelFactory.Create(DatabaseModel databaseModel, ModelReverseEngineerOptions options) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600021e+0xf6
   at Microsoft.EntityFrameworkCore.Scaffolding.Internal.ReverseEngineerScaffolder.ScaffoldModel(String connectionString, DatabaseModelFactoryOptions databaseOptions, ModelReverseEngineerOptions modelOptions, ModelCodeGenerationOptions codeOptions) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600023d+0xfe
   at Microsoft.EntityFrameworkCore.Design.Internal.DatabaseOperations.ScaffoldContext(String provider, String connectionString, String outputDir, String outputContextDir, String dbContextClassName, IEnumerable`1 schemas, IEnumerable`1 tables, String modelNamespace, String contextNamespace, Boolean useDataAnnotations, Boolean overwriteFiles, Boolean useDatabaseNames, Boolean suppressOnConfiguring, Boolean noPluralize) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600043e+0x11c
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.ScaffoldContextImpl(String provider, String connectionString, String outputDir, String outputDbContextDir, String dbContextClassName, IEnumerable`1 schemaFilters, IEnumerable`1 tableFilters, String modelNamespace, String contextNamespace, Boolean useDataAnnotations, Boolean overwriteFiles, Boolean useDatabaseNames, Boolean suppressOnConfiguring, Boolean noPluarlize) in Microsoft.EntityFrameworkCore.Design.dll:token 0x60003ed+0x32
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.ScaffoldContext.<>c__DisplayClass0_0.<.ctor>b__0() in Microsoft.EntityFrameworkCore.Design.dll:token 0x60006ed+0x0
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0() in Microsoft.EntityFrameworkCore.Design.dll:token 0x60006f3+0x0
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action) in Microsoft.EntityFrameworkCore.Design.dll:token 0x600066a+0xc
```

After this commit, scaffolding produces warnings for columns with missing type information  instead of crashing.

```
Could not find type mapping for column 'SalesLT.Address.StateProvince' with data type '???'. Skipping column.
Could not find type mapping for column 'SalesLT.Address.CountryRegion' with data type '???'. Skipping column.
Unable to scaffold the index 'IX_Address_AddressLine1_AddressLine2_City_StateProvince_PostalCode_CountryRegion'. The following columns could not be scaffolded: StateProvince,CountryRegion.
Unable to scaffold the index 'IX_Address_StateProvince'. The following columns could not be scaffolded: StateProvince.
```

Fixes dotnet#25729

src/EFCore.SqlServer/Scaffolding/Internal/SqlServerDatabaseModelFactory.cs

@@ -711,7 +711,7 @@ UNION ALL
             commandText += @"
 ) o
 JOIN [sys].[columns] AS [c] ON [o].[object_id] = [c].[object_id]
-JOIN [sys].[types] AS [tp] ON [c].[user_type_id] = [tp].[user_type_id]
+LEFT JOIN [sys].[types] AS [tp] ON [c].[user_type_id] = [tp].[user_type_id]
 LEFT JOIN [sys].[extended_properties] AS [e] ON [e].[major_id] = [o].[object_id] AND [e].[minor_id] = [c].[column_id] AND [e].[class] = 1 AND [e].[name] = 'MS_Description'
 LEFT JOIN [sys].[computed_columns] AS [cc] ON [c].[object_id] = [cc].[object_id] AND [c].[column_id] = [cc].[column_id]
 LEFT JOIN [sys].[default_constraints] AS [dc] ON [c].[object_id] = [dc].[parent_object_id] AND [c].[column_id] = [dc].[parent_column_id]";
@@ -744,7 +744,7 @@ UNION ALL
                     var columnName = dataRecord.GetFieldValue<string>("column_name");
                     var ordinal = dataRecord.GetFieldValue<int>("ordinal");
                     var dataTypeSchemaName = dataRecord.GetValueOrDefault<string>("type_schema");
-                    var dataTypeName = dataRecord.GetFieldValue<string>("type_name");
+                    var dataTypeName = dataRecord.GetValueOrDefault<string>("type_name") ?? "???";
                     var maxLength = dataRecord.GetValueOrDefault<int>("max_length");
                     var precision = dataRecord.GetValueOrDefault<int>("precision");
                     var scale = dataRecord.GetValueOrDefault<int>("scale");