Nullable and modernization part 6

[erikmav]

Adjusted nullability on some classes after applying the 3.0 binary to two existing projects that have nullable enabled. Changed nullable warnings to errors.

[erikmav]

For #124

[erikmav]

@FreeAndNil the ugliest adaptations I had to make with the new binary:

• Level? on LoggingEvent and its internal data structure required adding a bunch of null checks, especially where there are comparisons of levels. Lots of these in 2 repos. It would be ideal to make that non-nullable and ensure it's set to some default like Level.Off or similar.
• Repository? on Logger required adding null checks. Generally speaking this is never null in practice, but I don't see what default to set it to.

[erikmav]

@FreeAndNil I just tried setting LoggingEvent.Level to non-nullable and set to Level.All. Problem is that null is converted into different policies by different appenders (Debug, Info, or All) so a blanket All policy does not work. Seems like it needs to be left nullable for back compat.