Avoid NullPointerExceptions in org.apache.logging.log4j.core.config.AbstractConfiguration for null arguments to appenders

Avoid NPEs in the class org.apache.logging.log4j.core.config.AbstractConfiguration for null arguments to appenders which uses ConcurrentHashMap which in turn throws NullPointerException for null keys and values.

I have edge cases where null trickles down into these Log4j APIs.