Result of `SignatureHelpHandlerBase` is not nullable

[steveRoll-git]

According to the LSP spec, the response to textDocument/signatureHelp can be SignatureHelp | null.
But, the return type of SignatureHelpHandlerBase.Handle is Task<SignatureHelp>, which does not allow null.

I can make a PR that addresses this, I just want to make sure whether this is intentional or not.

[CppCXY]

I checked the LSP specification. The signatureHelp request now indeed supports null as a return value, so you can fix this issue via a PR.