Add trimToNull to Strings utility class.

  - Made class final.
  - Added private constructor.
  - Fixed javadoc to refer to correct class.
  - Removed @since tags from javadoc (wrong version references).

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1597502 13f79535-47bb-0310-9956-ffa450edef68