Remove default broken URL added to new courses

New courses created in studio currently have a broken default image added by default.
By removing this URL at creation, it enables the default image set in LMS to display: https://github.com/edx/edx-platform/pull/8282

common/lib/xmodule/xmodule/course_module.py

@@ -681,7 +681,7 @@ class CourseFields(object):
         ),
         scope=Scope.settings,
         # Ensure that courses imported from XML keep their image
-        default="images_course_image.jpg"
+        default=""
     )
     issue_badges = Boolean(
         display_name=_("Issue Open Badges"),