Fix unexpected null in principal columns of foreign key

[carlreinke]

MigrationsSqlGenerator expects PrincipalColumns to be null, not a null element.

[ajcvickers]

@carlreinke Thanks for your interest in fixing this. Two comments. First, please target this PR to the main branch. Fixes can be considered for porting back to a patch release, but we need to have a solid fix in main first.

Second, please add tests that fail before this change and pass afterwards. This will verify that the issue is really fixed and help prevent regressions in the future.

[carlreinke]

@ajcvickers Where are the tests for MigrationBuilder?

[AndriySvyryd]

https://github.com/dotnet/efcore/blob/main/test/EFCore.Design.Tests/Migrations/Design/CSharpMigrationOperationGeneratorTest.cs

[carlreinke]

There were indeed more changes needed to make it work end-to-end.

I tidied up the AddForeignKeyOperation tests so that the ordering of properties, parameters, and assertions are consistent, but I can put them back if you prefer a smaller diff.

[AndriySvyryd]

Thanks for your contribution