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

And add checkstyle test for lines containing only whitespace characters.

smack-tcp/src/test/java/org/jivesoftware/smack/tcp/PacketWriterTest.java

@@ -102,7 +102,7 @@ public class PacketWriterTest {
             t.notify();
         }
     }
-    
+
     public class BlockingStringWriter extends Writer {
         @Override
         public void write(char[] cbuf, int off, int len) throws IOException {