added isEmpty method

util/src/main/java/de/muehlencord/shared/util/StringUtil.java

@@ -87,4 +87,13 @@ public abstract class StringUtil {
         returnValue = returnValue.trim();
         return returnValue;
     }
+
+    /**
+     * returns true, if given string is either null or a blank string
+     * @param s the string to check
+     * @return true, if s is either null or s.equals("")
+     */
+    public static boolean isEmpty(String s) {
+        return (s != null) && (!s.equals (""));
+    }
 }