Ignore dot-source commands when finding for Pester document symbols

This change fixes an issue in the PesterDocumentSymbolProvider where
CommandAsts for dot-sourced scripts were being evaluated by the symbol
provider and causing a crash when GetCommandName() returned null.  This
change causes dot-sourced commands to now be skipped and also skips any
CommandAst which returns null for GetCommandName().

Resolves PowerShell/vscode-powershell#859

src/PowerShellEditorServices/Symbols/PesterDocumentSymbolProvider.cs

@@ -33,6 +33,7 @@ IEnumerable<SymbolReference> IDocumentSymbolProvider.ProvideDocumentSymbols(
 
                 return
                     commandAst != null &&
+                    commandAst.InvocationOperator != TokenKind.Dot &&
                     PesterSymbolReference.GetCommandType(commandAst.GetCommandName()).HasValue &&
                     commandAst.CommandElements.Count >= 2;
             },
@@ -144,6 +145,11 @@ internal PesterSymbolReference(
 
         internal static PesterCommandType? GetCommandType(string commandName)
         {
+            if (commandName == null)
+            {
+                return null;
+            }
+
             switch (commandName.ToLower())
             {
                 case "describe":