org.apache.logging.log4j.core.LogEvent.getContextMap() should never return null. Remove the getContextMap(String) API (aka getContextMapValue(String). The FlumeEvent is a little different and can indeed hold a null context map. All tests pass from Maven.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1595698 13f79535-47bb-0310-9956-ffa450edef68