add isNodeEmpty helper method

packages/core/src/helpers/isNodeEmpty.ts

@@ -0,0 +1,8 @@
+import { Node as ProseMirrorNode } from 'prosemirror-model'
+
+export default function isNodeEmpty(node: ProseMirrorNode): boolean {
+  const defaultContent = node.type.createAndFill()?.toJSON()
+  const content = node.toJSON()
+
+  return JSON.stringify(defaultContent) === JSON.stringify(content)
+}