fix: work item assignee update validation (#6704)

Author: prateekshourya29

apiserver/plane/app/serializers/issue.py

@@ -217,7 +217,7 @@ def update(self, instance, validated_data):
         updated_by_id = instance.updated_by_id
 
         valid_assignee_ids = self.get_valid_assignees(assignees, project_id)
-        if valid_assignee_ids is not None and len(valid_assignee_ids):
+        if valid_assignee_ids is not None:
             IssueAssignee.objects.filter(issue=instance).delete()
             try:
                 IssueAssignee.objects.bulk_create(