Update null handling.

  - Don't allow null Marker names. They won't work anyway due to the use of ConcurrentHashMap.
  - Update methods to throw NullPointerException instead of IllegalArgumentException to match updated javadocs.
  - Simplify equals and hashCode based on guaranteed non-nullity of marker name.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1589425 13f79535-47bb-0310-9956-ffa450edef68