Add isEmpty to AbstractMmlEmptyNode

Author: dpvc

ts/core/MmlTree/MmlNode.ts

@@ -1251,6 +1251,13 @@ export abstract class AbstractMmlEmptyNode
     return false;
   }
 
+  /**
+   * @returns {boolean}   Is empty
+   */
+  public get isEmpty(): boolean {
+    return true;
+  }
+
   /**
    * @returns {boolean}  Not embellished
    */