Fix parameter initialisation in test helper

Addresses an issue identified by sonarqube during implementing BAR-10
where a variable is initialised to an empty list within a function
definition.

tests/test_barman_cloud_backup_delete.py

@@ -184,12 +184,14 @@ def _get_sorted_files_for_backup(self, backup_metadata, backup_id):
             files_for_backup += sorted(additional_files)
         return files_for_backup
 
-    def _create_catalog(self, backup_metadata, wals=[]):
+    def _create_catalog(self, backup_metadata, wals=None):
         """
         Create a mock CloudBackupCatalog from the supplied data so that we can provide
         a work-alike CloudBackupCatalog to the code-under-test without also including
         the CloudBackupCatalog logic in the tests.
         """
+        if wals is None:
+            wals = []
         # Copy so that we don't affect the state the tests are using when the
         # code under test removes backups from the mock catalog
         backup_state = backup_metadata.copy()