add isEmpty method, fix #28

packages/core/src/Editor.ts

@@ -387,6 +387,13 @@ export class Editor extends EventEmitter {
     return getHtmlFromFragment(this.state.doc, this.schema)
   }
 
+  /**
+   * Check if there is no content.
+   */
+  public isEmpty() {
+    return !this.state.doc.textContent.length
+  }
+
   /**
    * Destroy the editor.
    */