Fix NRE when invoking completion in empty document

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()
     {