Treat null Logger as no-op in ctx()

[merlimat]

Summary

• LoggerBuilder.ctx(Logger) and Event.ctx(Logger) now return early when passed null instead of throwing NullPointerException.
• Aligns the null path with the existing empty-logger no-op behavior, useful when callers pass an optional context logger that may be unset.
• Javadoc on both interface methods updated to document the null behavior.

Test plan

• New tests ctxWithNullLoggerIsNoOp and eventCtxWithNullLoggerIsNoOp added alongside the existing empty-logger tests.
• ./gradlew test passes.