's;^\s+$;;' on all source files

And add checkstyle test for lines containing only whitespace characters.

smack-im/src/test/java/org/jivesoftware/smack/roster/RosterTest.java

@@ -422,7 +422,7 @@ public class RosterTest extends InitSmackIm {
                 assertSame("Shouldn't provide an empty group element!",
                         0,
                         item.getGroupNames().size());
-                
+
             }
         };
         serverSimulator.start();