Fix NRE when invoking completion in empty document (#10610)

* Fix NRE when invoking completion in empty document

In empty document completion context will have RazorDocumentSyntax as Owner. RazorDoucmentSyntax has null as the parent, which makes sense. However compiler thinks we can't have a null here, while obviously we do.

I'm adding a null check and a test for this case. We would probably further discuss the implications here. I tried to track down how we get null in a non-nullable field, but it's a bit confusing since the class gets generated.

* Move RazorDocumentSyntax special case inside switch per CR suggestion

* Fixing build

Author: alexgav

src/Razor/src/Microsoft.AspNetCore.Razor.LanguageServer/Completion/TagHelperCompletionProvider.cs

@@ -48,14 +48,16 @@ public ImmutableArray<RazorCompletionItem> GetCompletionItems(RazorCompletionCon
         if (owner is null)
         {
             Debug.Fail("Owner should never be null.");
-            return ImmutableArray<RazorCompletionItem>.Empty;
+            return [];
         }
 
         owner = owner switch
         {
             // This provider is trying to find the nearest Start or End tag. Most of the time, that's a level up, but if the index the user is typing at
             // is a token of a start or end tag directly, we already have the node we want.
             MarkupStartTagSyntax or MarkupEndTagSyntax or MarkupTagHelperStartTagSyntax or MarkupTagHelperEndTagSyntax or MarkupTagHelperAttributeSyntax => owner,
+            // Invoking completion in an empty file will give us RazorDocumentSyntax which always has null parent
+            RazorDocumentSyntax => owner,
             // Either the parent is a context we can handle, or it's not and we shouldn't show completions.
             _ => owner.Parent
         };
@@ -118,7 +120,7 @@ static bool InOrAtEndOfAttribute(SyntaxNode attributeSyntax, int absoluteIndex)
         }
 
         // Invalid location for TagHelper completions.
-        return ImmutableArray<RazorCompletionItem>.Empty;
+        return [];
     }
 
     private ImmutableArray<RazorCompletionItem> GetAttributeCompletions(

src/Razor/test/Microsoft.AspNetCore.Razor.LanguageServer.Test/Completion/TagHelperCompletionProviderTest.cs

@@ -89,6 +89,23 @@ public void GetCompletionAt_AtEmptyTagName_ReturnsCompletions()
             completion => Assert.Equal("test2", completion.InsertText));
     }
 
+    [Fact]
+    public void GetCompletionAt_InEmptyDocument_ReturnsEmptyCompletionArray()
+    {
+        // Arrange
+        var service = new TagHelperCompletionProvider(RazorTagHelperCompletionService, TestRazorLSPOptionsMonitor.Create());
+        var context = CreateRazorCompletionContext(
+            "$$",
+            isRazorFile: true,
+            tagHelpers: DefaultTagHelpers);
+
+        // Act
+        var completions = service.GetCompletionItems(context);
+
+        // Assert
+        Assert.Empty(completions);
+    }
+
     [Fact]
     public void GetCompletionAt_OutsideOfTagName_DoesNotReturnCompletions()
     {