sso/backends: remove_* must not change the user (#13430)

_update_m2m_from_groups must return None if remove_* is false or empty,
because None indicates that the user permissions will not be changed.

related #13429

awx/sso/backends.py

@@ -354,7 +354,9 @@ def _update_m2m_from_groups(ldap_user, opts, remove=True):
                 continue
             if ldap_user._get_groups().is_member_of(group_dn):
                 return True
-    return False
+    if remove:
+        return False
+    return None
 
 
 @receiver(populate_user, dispatch_uid='populate-ldap-user')