Hide the "Edit" button if no eXe URL is provided

[ignaciogros]

Leave the eXe URL field empty when configuring the plugin, and then go to an activity. You'll still see the "Edit with eXe" button, which should not be there until that URL is provided.

[pabloamayab]

This issue has been tested in Moodle 4.5.1 and it's ok