8215401: Add isEmpty default method to CharSequence

Reviewed-by: jlaskey, rriggs, jjg, alanb, smarks, darcy

src/java.base/share/classes/java/lang/String.java

@@ -684,6 +684,7 @@ public final class String
      *
      * @since 1.6
      */
+    @Override
     public boolean isEmpty() {
         return value.length == 0;
     }