8215401: Add isEmpty default method to CharSequence

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

src/java.base/share/classes/java/nio/X-Buffer.java.template

@@ -1645,6 +1645,18 @@ public abstract class $Type$Buffer
         return remaining();
     }
 
+    /**
+     * Returns {@code true} if this character buffer is empty.
+     *
+     * @return {@code true} if there are {@code 0} remaining characters,
+     *         otherwise {@code false}
+     *
+     * @since 15
+     */
+    public final boolean isEmpty() {
+        return remaining() == 0;
+    }
+
     /**
      * Reads the character at the given index relative to the current
      * position.