SignatureHelp and CompletionItem spec fix

[TylerLeonhardt]

Fixes PowerShell/PowerShellEditorServices#1324
Fixes PowerShell/PowerShellEditorServices#1325

Tags on CompletionItem should be optional and
Signatures on SignatureHelp should not be null

[codecov]

Codecov Report

Merging #258 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #258   +/-   ##
=======================================
  Coverage   62.06%   62.07%           
=======================================
  Files         383      384    +1     
  Lines       11489    11490    +1     
  Branches     1361     1361           
=======================================
+ Hits         7131     7132    +1     
  Misses       3788     3788           
  Partials      570      570           

Impacted Files	Coverage Δ
src/Protocol/Models/SignatureHelp.cs	100.00% <100.00%> (ø)

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