Add isEmpty to AccountId (#583)

packages/types/src/AccountId.ts

@@ -40,6 +40,13 @@ export default class AccountId extends U8aFixed {
     return value;
   }
 
+  /**
+   * @description Returns true if the type wraps a non-encodable value
+   */
+  get isEmpty (): boolean {
+    return this.length !== 32;
+  }
+
   /**
    * @description Converts the Object to JSON, typically used for RPC transfers
    */