Bug 1541121: [try-staging] Simplify the logic for getting partial update configration; r=aki

The original code was converting to json, then comparing against `{}`. This switches things
around so that json is only generated where it is directly used a json.

Differential Revision: https://phabricator.services.mozilla.com/D24833

--HG--
extra : moz-landing-system : lando

taskcluster/taskgraph/actions/release_promotion.py

@@ -252,17 +252,17 @@ def release_promotion_action(parameters, graph_config, input, task_group_id, tas
             )
 
     if promotion_config.get('partial-updates', False):
-        partial_updates = json.dumps(input.get('partial_updates', {}))
-        if partial_updates == "{}":
+        partial_updates = input.get('partial_updates', {})
+        if not partial_updates:
             raise Exception(
                 "`partial_updates` property needs to be provided for `{}`"
                 "target.".format(release_promotion_flavor)
             )
         balrog_prefix = product.title()
-        os.environ['PARTIAL_UPDATES'] = partial_updates
+        os.environ['PARTIAL_UPDATES'] = json.dumps(partial_updates)
         release_history = populate_release_history(
             balrog_prefix, parameters['project'],
-            partial_updates=input['partial_updates']
+            partial_updates=partial_updates
         )
 
     target_tasks_method = promotion_config['target-tasks-method'].format(