fix scale down check for sharding (#310)

## Issue
#308 
occassionally after setting something in the peer data bag, reading the
data fails

## Solution
check the boolean

src/charm.py

@@ -1317,9 +1317,9 @@ def check_relation_broken_or_scale_down(self, event: RelationDepartedEvent) -> N
         Relation departed and relation broken events occur during scaling down or during relation
         removal, only relation departed events have access to metadata to determine which case.
         """
-        self.set_scaling_down(event)
+        scaling_down = self.set_scaling_down(event)
 
-        if self.is_scaling_down(event.relation.id):
+        if scaling_down:
             logger.info(
                 "Scaling down the application, no need to process removed relation in broken hook."
             )
@@ -1334,12 +1334,14 @@ def has_departed_run(self, rel_id: int) -> bool:
         rel_departed_key = self._generate_relation_departed_key(rel_id)
         return rel_departed_key in self.unit_peer_data
 
-    def set_scaling_down(self, event: RelationDepartedEvent) -> None:
+    def set_scaling_down(self, event: RelationDepartedEvent) -> bool:
         """Sets whether or not the current unit is scaling down."""
         # check if relation departed is due to current unit being removed. (i.e. scaling down the
         # application.)
         rel_departed_key = self._generate_relation_departed_key(event.relation.id)
-        self.unit_peer_data[rel_departed_key] = json.dumps(event.departing_unit == self.unit)
+        scaling_down = json.dumps(event.departing_unit == self.unit)
+        self.unit_peer_data[rel_departed_key] = scaling_down
+        return scaling_down
 
     @staticmethod
     def _generate_relation_departed_key(rel_id: int) -> str: