Bug 1726764 - let release promotion work without partials for staging releases. r=releng-reviewers,jmaher,bhearsum

"This is nightly so we don't know what partials we need" is not the same
as "we're making a release and we know we don't want partials".  In one
case (when called from get_decision_parameters) partial_updates is None,
in the other (called from release_promotion_action) it's an empty dict.
The latter case still enforces non-empty partial_updates for production,
but for staging it's not really necessary and falling back to nightly
behaviour in populate_release_history is just wrong.

Differential Revision: https://phabricator.services.mozilla.com/D127228

taskcluster/taskgraph/util/partials.py

@@ -188,7 +188,7 @@ def populate_release_history(
 ):
     # Assuming we are using release branches when we know the list of previous
     # releases in advance
-    if partial_updates:
+    if partial_updates is not None:
         return _populate_release_history(
             product, branch, partial_updates=partial_updates
         )