Bugfix – handle missing location

The location, logEvent.getSource(), is not always available, eg in the case of async loggers.

Any attempt from LogStashJSONLayout to write location information in this situation results in a NPE.

This pull request is based ontop of #8
(it needs be merged afterwards, and only commit https://github.com/finn-no/log4j2-logstash-jsonevent-layout/commit/b9d4ba4b55adb332502c6d45e992db0fc7c7920dneeds be evaluated)