Fix a mutable default function arg noticed when digging through history API code.

lib/galaxy/webapps/galaxy/api/histories.py

@@ -507,7 +507,7 @@ class HistoriesController(BaseAPIController, ExportsHistoryMixin, ImportsHistory
         return self.serve_ready_history_export(trans, jeha)
 
     @expose_api
-    def get_custom_builds_metadata(self, trans, id, payload={}, **kwd):
+    def get_custom_builds_metadata(self, trans, id, payload=None, **kwd):
         """
         GET /api/histories/{id}/custom_builds_metadata
         Returns meta data for custom builds.
@@ -515,6 +515,8 @@ class HistoriesController(BaseAPIController, ExportsHistoryMixin, ImportsHistory
         :param id: the encoded history id
         :type  id: str
         """
+        if payload is None:
+            payload = {}
         history = self.history_manager.get_accessible(self.decode_id(id), trans.user, current_history=trans.history)
         installed_builds = []
         for build in glob.glob(os.path.join(trans.app.config.len_file_path, "*.len")):