[FIX] web_editor: don't run code when input is emptied

Before this commit:
  - Edit mode, open image dialog
  - Upload an image to the media dialog
  - Reclick on Upload but don't select an image, close the browser image
    selector
  - This will trigger the input change event, as the input was emptied

This is needed since in later commit, `_addData()` will do some code outside
of the `files` loop, which is useless if there is no files selected.
In thise case, the issue would be that it would open an empty toastr.

task-2345082
Coming from #65828
Closes #68326

closes odoo/odoo#68340

X-original-commit: 94c45412714c1263788b2567ce3c9581453b2cd4
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/js/wysiwyg/widgets/media.js

@@ -557,13 +557,18 @@ var FileWidget = SearchableMediaWidget.extend({
      * @private
      * @returns {Promise}
      */
-    _addData: function () {
+    async _addData() {
+        let files = this.$fileInput[0].files;
+        if (!files.length) {
+            // Case if the input is emptied, return resolved promise
+            return;
+        }
+
         var self = this;
         var uploadMutex = new concurrency.Mutex();
 
         // Upload the smallest file first to block the user the least possible.
-        var files = _.sortBy(this.$fileInput[0].files, 'size');
-
+        files = _.sortBy(files, 'size');
         _.each(files, function (file) {
             // Upload one file at a time: no need to parallel as upload is
             // limited by bandwidth.