Null inputs replaced

[karstenspang]

At least two places, nulls are replaced by other values:

• When a LoggingEvent is constructed with arguments, nulls are replaced by Option.empty()
• If a null value is inserted in the MDC, it replaced by the string "null".

I am not sure what was the rationale for implementing it like this. For the first case, it may beacuse the original author wanted to use the Guava ImmutableList. But it also seems that he had a dislike for null in general.
If you just compare logging events, the first has no impact, but if you are testing MDC functionality, the second could be annoying. In particular, the assertion failure message shows null in both the expected and the actual value.

I can quickly make a PR to change this if you think it is a good idea.

[valfirst]

not sure what impact will be, but let's try, please proceed with PR