[IMP] website_slides: upload document doesn't rewrite title everytime

Before when you uploaded a document, the title was set to the filename even when the field was
set by the user before. Now, the field title is set to the filename only when it is
empty.

task-2161391

addons/website_slides/static/src/js/slides_upload.js

@@ -516,7 +516,9 @@ var SlideUploadDialog = Dialog.extend({
         if (!preventOnchange) {
             var input = file.name;
             var inputVal = input.substr(0, input.lastIndexOf('.')) || input;
-            this._formSetFieldValue('name', inputVal);
+            if (this._formGetFieldValue('name') === "") {
+                this._formSetFieldValue('name', inputVal);
+            }
         }
     },
     _onChangeSlideUrl: function (ev) {