Only overwrite empty dominant values, but no longer blank ones

This closes codehaus-plexus#212

Author: kwin

src/main/java/org/codehaus/plexus/util/xml/Xpp3DomUtils.java

@@ -282,6 +282,6 @@ public static boolean isNotEmpty( String str )
 
     public static boolean isEmpty( String str )
     {
-        return ( str == null || str.trim().length() == 0 );
+        return ( str == null || str.length() == 0 );
     }
 }