Allow to set an attribute to the empty list

mpcdf_enable_repositories.py

@@ -77,7 +77,7 @@ def do_mpcdf_enable_repositories(self, subcmd, opts, *args):
         mpcdf_common.set_attribute_values(api_url, project, package, "MPCDF:enable_repositories", opts.set.split(","))
 
         def set_or_remove(flag, attribute_name):
-            if flag:
+            if flag is not None:
                 print("Setting attribute", attribute_name, "to", flag)
                 mpcdf_common.set_attribute_values(api_url, project, package, attribute_name, flag.split(","))
             elif mpcdf_common.has_attribute(api_url, project, package, attribute_name):