support empty body

packages/app/src/components/PageEditor/LinkEditModal.jsx

@@ -37,7 +37,7 @@ class LinkEditModal extends React.PureComponent {
       linkInputValue: '',
       labelInputValue: '',
       linkerType: Linker.types.markdownLink,
-      markdown: '',
+      markdown: null,
       previewError: '',
       permalink: '',
       isPreviewOpen: false,
@@ -152,7 +152,7 @@ class LinkEditModal extends React.PureComponent {
   async setMarkdown() {
     const { t } = this.props;
     const path = this.state.linkInputValue;
-    let markdown = '';
+    let markdown = null;
     let permalink = '';
     let previewError = '';
 

packages/app/src/components/PageEditor/PreviewWithSuspense.jsx

@@ -5,7 +5,7 @@ import Preview from './Preview';
 import { withLoadingSppiner } from '../SuspenseUtils';
 
 function PagePreview(props) {
-  if (props.markdown === '') {
+  if (props.markdown == null) {
     if (props.error !== '') {
       return props.error;
     }