Show default bookmark title for synced bookmarks

Fix #7291

Test plan:
1. open pyramid 0, enable sync, bookmark something.
2. open pyramid 1, sync to pyramid 0, wait for the synced bookmark to appear.
3. click 'edit bookmark...' on the synced bookmark in pyramid 1. the title field should not be empty.

app/renderer/components/addEditBookmarkHanger.js

@@ -36,7 +36,8 @@ class AddEditBookmarkHanger extends ImmutableComponent {
       : (typeof location === 'string' && location.trim().length > 0)
   }
   get displayBookmarkName () {
-    if (this.props.currentDetail.get('customTitle') !== undefined) {
+    const customTitle = this.props.currentDetail.get('customTitle')
+    if (customTitle !== undefined && customTitle !== '') {
       return this.props.currentDetail.get('customTitle')
     }
     return this.props.currentDetail.get('title') || ''