Added some additional nullable annotations

[david-driscoll]

No description provided.

[codecov]

Codecov Report

Merging #380 into master will increase coverage by 0.03%.
The diff coverage is 85.00%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   75.23%   75.26%   +0.03%     
==========================================
  Files         556      555       -1     
  Lines       13034    13047      +13     
  Branches     1190     1193       +3     
==========================================
+ Hits         9806     9820      +14     
+ Misses       3228     3227       -1     

Impacted Files	Coverage Δ
...ient/Configuration/ChainedConfigurationProvider.cs	75.86% <ø> (ø)
...ient/Configuration/ChainedConfigurationProvider.cs	17.24% <ø> (ø)
...Client/Configuration/ChainedConfigurationSource.cs	0.00% <0.00%> (ø)
...rver/Configuration/ChainedConfigurationProvider.cs	17.24% <ø> (ø)
...Server/Configuration/ChainedConfigurationSource.cs	0.00% <0.00%> (ø)
src/JsonRpc/NoopResponseRouter.cs	0.00% <0.00%> (ø)
src/JsonRpc/RequestRouterBase.cs	100.00% <ø> (ø)
src/Protocol/LanguageProtocolDelegatingHandlers.cs	41.57% <ø> (ø)
src/Protocol/Models/DocumentSelector.cs	78.94% <ø> (ø)
src/Protocol/Models/Position.cs	63.15% <ø> (ø)
... and 23 more

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