#124 LoggingEvent.Level not nullable

[FreeAndNil]

#124 LoggingEvent.Level not nullable
@erikmav what do you think about this approach?

[erikmav]

In the 2.x codebase a null level was handled as an allowed option, and some formatters use default behaviors on a null value (search for level is null across the codebase). IOW this is probably a back compat problem where we might break people. Maybe that's OK in v3... which means we need if (level is null) { throw new ArgumentNullException(nameof(level)); } anywhere we're receiving a level from the user.