Added request and response processors that ensures they are not null

[david-driscoll]

This is a simple change to ensure that null requests and null responses (through the MediatR pipeline) throws an error.

[codecov]

Codecov Report

Merging #265 into master will increase coverage by 0.07%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   60.88%   60.96%   +0.07%     
==========================================
  Files         350      352       +2     
  Lines       10328    10336       +8     
  Branches     1167     1169       +2     
==========================================
+ Hits         6288     6301      +13     
+ Misses       3673     3664       -9     
- Partials      367      371       +4     

Impacted Files	Coverage Δ
...JsonRpc/Pipelines/RequestMustNotBeNullProcessor.cs	60.00% <60.00%> (ø)
...sonRpc/Pipelines/ResponseMustNotBeNullProcessor.cs	60.00% <60.00%> (ø)
src/JsonRpc/ServiceCollectionExtensions.cs	88.88% <100.00%> (-1.12%)	⬇️
src/Server/LanguageServer.Shutdown.cs	62.06% <0.00%> (ø)
src/JsonRpc/RequestRouterBase.cs	85.08% <0.00%> (+4.38%)	⬆️
src/JsonRpc/Server/Messages/InvalidParams.cs	50.00% <0.00%> (+50.00%)	⬆️
src/JsonRpc/Server/InvalidParametersException.cs	50.00% <0.00%> (+50.00%)	⬆️

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