fix(graphql-language-service): return null instead of empty contents array when hover results are empty

[joshbode]

I recently submitted a (rejected) PR in Neovim because LSPs including graphiql and vtsls are returning empty hover `contents` arrays when there is no result, which is almost always the case for graphiql since it would only ever be expected to produce hover results for embedded #graphql query strings.

The effect of this is that graphiql is showing up in the Neovim LSP hover summary every time a hover request is made, even when there are no results, at locations in the code where results from graphiql would never be expected:

Since the LSP spec allows a hover request to return `null`, this change is to align with the LSP spec to return an empty result (null) when there are no results instead of an empty contents array.

Thank you for the GraphiQL language server :)

Related PR: yioneko/vtsls#248

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🦋 Changeset detected

Latest commit: a4b5ab1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
graphql-language-service-server	Patch

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[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: joshbode / name: Josh Bode (6922c10)
• ✅ login: dimaMachina / name: Dimitri POSTOLOV (a4b5ab1)

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.93%. Comparing base (03171d5) to head (6922c10).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ql-language-service-server/src/MessageProcessor.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3913   +/-   ##
=======================================
  Coverage   63.93%   63.93%           
=======================================
  Files          35       35           
  Lines        3086     3086           
  Branches      950      948    -2     
=======================================
  Hits         1973     1973           
  Misses       1108     1108           
  Partials        5        5           

Files with missing lines	Coverage Δ
...ql-language-service-server/src/MessageProcessor.ts	89.18% <50.00%> (ø)

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