BOM-1417 (#23476)

Fixing management command.

Django PR removed force_str method: https://github.com/django/django/commit/dc8834cad41aa407f402dc54788df3cd37ab3e22#diff-55346957c9b0473492d19a390a7adb47L36

None is not working now.

cms/djangoapps/contentstore/management/commands/export_olx.py

@@ -60,7 +60,7 @@ class Command(BaseCommand):
 
         export_course_to_tarfile(course_key, filename)
 
-        results = self._get_results(filename) if pipe_results else None
+        results = self._get_results(filename) if pipe_results else ''
 
         self.stdout.write(results, ending="")