No completion items are returned if invoking completion in an empty Razor document

[alexgav]

If you add an empty .razor or .cshtml file and try to invoke completion using Ctrl+Space or Ctrl+J, you get no results.

We should get HTML tags and tag helpers (and snippets)

If the document is completely empty (zero length, not even whitespace), the issue is that RazorHtmlWriter doesn't produce mapping spans for zero-length tokens, so position doesn't map to HTML. See https://github.com/dotnet/razor/blob/7b511df1710c54ab1a36ea5e422c7edd7bb29d20/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/RazorHtmlWriter.cs#L198C13-L198C35

Correct fix would be to produce a mapping span for zero-length token at least for the special case of a completely empty (zero-length) document. Modifying compiler is a bit risky though, so maybe there is a less impactful workaround.

Also a completely empty razor file is probably not that common in practice. I would say this is probably a lower priority bug, but something we should keep track of.

Also see #10610 for some discussion and a related fix.