Add defensive null-checks for MDC in order work around logback awesomeness

[bbrehm]

Some versions of logback return null-pointer on getCopyOfContextMap() and then choke on setContextMap(null).

Great API design all around! Good on us that we're using lol4j, that one doesn't choke on null-pointers, and even supports hot-loading of plugin functionality.

Thanks to BigDataDave for reporting the issue on discord.