Cleanup various bits of Google.Protobuf#6674

Merged

jtattermusch merged 6 commits intoprotocolbuffers:masterfrom ObsidianMinor:csharp/general-cleanup

Dec 5, 2019

+20-19

Commits on Nov 19, 2019

Add some missing null-checks
ObsidianMinor
committed

Commits on Nov 20, 2019

Commits on Dec 2, 2019

Add comment and Assert.DoesNotThrow to RequiredFieldsNoThrow
ObsidianMinor
committed

Commits on Dec 3, 2019

Make test comment a summary
ObsidianMinor
committed