Xpp3DomUtils#mergeIntoXpp3Dom() must not override the dominant value in case it is empty

[kwin]

Although that behaviour is also stated in its javadoc it is highly unexpected. Rather a dominant empty value element should take precedence over the same named recessive element value. This would be necessary for https://issues.apache.org/jira/browse/MNG-6434 to work as expected.

[michael-o]

...an empty string is still a valid string...unless you use Oracle :-)