Fix nullable warnings in guard class

[mic-max]

Fixes this warning

Related open-telemetry/opentelemetry-dotnet-contrib#332

Changes

Made the parameters will null as default values nullable fields.
Added #nullable enable to the file so that I could use nullable fields.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #

[codecov]

Codecov Report

Merging #3236 (81a73c1) into main (0741a9d) will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3236      +/-   ##
==========================================
+ Coverage   85.29%   85.56%   +0.26%     
==========================================
  Files         261      261              
  Lines        9385     9385              
==========================================
+ Hits         8005     8030      +25     
+ Misses       1380     1355      -25     

Impacted Files	Coverage Δ
src/OpenTelemetry.Api/Internal/Guard.cs	0.00% <ø> (ø)
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	92.34% <0.00%> (+3.60%)	⬆️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️
...tation/OpenTelemetryProtocolExporterEventSource.cs	85.00% <0.00%> (+10.00%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	77.27% <0.00%> (+40.90%)	⬆️
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	78.57% <0.00%> (+42.85%)	⬆️