Use Java 6 API: String#isEmpty().

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1594970 13f79535-47bb-0310-9956-ffa450edef68
Adjust test for change in source.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1594986 13f79535-47bb-0310-9956-ffa450edef68