fix thumbnails disabling publish (#7714)

ui/component/publishForm/view.jsx

@@ -208,7 +208,6 @@ function PublishForm(props: Props) {
     isNameValid(name) &&
     title &&
     bid &&
-    thumbnail &&
     !bidError &&
     !emptyPostError &&
     !(thumbnailError && !thumbnailUploaded) &&

ui/component/selectThumbnail/view.jsx

@@ -106,7 +106,7 @@ function SelectThumbnail(props: Props) {
         __('This will be visible in a few minutes after you submit this form.')}
       <img
         style={{ display: 'none' }}
-        src={thumbnail}
+        src={thumbnail || ThumbnailMissingImage}
         alt={__('Thumbnail Preview')}
         onError={() => {
           if (updateThumbnailParams) {