Fixed product card throwing "Cannot read properties of undefined (reading 'url')" errors

closes https://github.com/TryGhost/Team/issues/2295

Problem:
- `<GhUploader>` is not yet converted to an Octane component so it's arguments are not read-only
- when a file is selected it sets it's `files` property which in turn updates the tracked `files` property that was passed in, and then again updates it to an empty file list when the input field is cleared
- that tracked property was never cleared once the product image was uploaded resulting in a "re-upload" attempt with an empty file list every time the product card was put back into edit mode

Fix:
- added a guard in `<GhUploader>` so it doesn't try to upload an empty file list if one is passed in as an attribute
- added a reset of the tracked `files` property in the product card once the image upload is complete

ghost/admin/app/components/gh-uploader.js

@@ -113,8 +113,9 @@ export default Component.extend({
         }
 
         // if we have new files, validate and start an upload
-        let files = this.files;
-        this._setFiles(files);
+        if (this.files?.length) {
+            this._setFiles(this.files);
+        }
     },
 
     actions: {

ghost/admin/lib/koenig-editor/addon/components/koenig-card-product.js

@@ -267,6 +267,8 @@ export default class KoenigCardProductComponent extends Component {
             this._productImageWidth = null;
             this._productImageHeight = null;
         });
+
+        this.files = null;
     }
 
     /**