Missing null branch on when statements

[Bertk]

Prepare benchmark tests for coverlet.core (currently uses InProcessToolchain) => could be used to monitor performance regressions

• add benchmarks tests
• fix build warnings
• partial update of nuget packages

Results for master branch (small updates for dotnet.benchmark)

[Bertk]

Results for coverlet 6.0.1 release (small updates for dotnet.benchmark)

[Bertk]

@daveMueller @MarcoRossignoli Please send me your feedback whether this should added.

I use coverlet.core.benchmark.tests to identify the commit introducing the coverlet 6.0.2 performance regression (see PR #1689).