🚨(backend) fix linter warnings

Recent updates of dev/ruff and dev/pylint dependencies led
to new linting warnings.

Pylint 3.2.0 introduced a new check `possibly-used-before-assignment`,
which ensures variables are defined regardless of conditional statements.

Some if/else branches were missing defaults. These have been fixed.

src/backend/core/tests/rooms/test_api_rooms_create.py

@@ -1,6 +1,7 @@
 """
 Test rooms API endpoints in the Meet core app: create.
 """
+
 import pytest
 from rest_framework.test import APIClient
 

src/backend/core/tests/rooms/test_api_rooms_delete.py

@@ -1,6 +1,7 @@
 """
 Test rooms API endpoints in the Meet core app: delete.
 """
+
 import pytest
 from rest_framework.test import APIClient
 

src/backend/core/tests/rooms/test_api_rooms_list.py

@@ -1,6 +1,7 @@
 """
 Test rooms API endpoints in the Meet core app: list.
 """
+
 from unittest import mock
 
 import pytest

src/backend/core/tests/rooms/test_api_rooms_retrieve.py

@@ -1,6 +1,7 @@
 """
 Test rooms API endpoints in the Meet core app: retrieve.
 """
+
 import random
 from unittest import mock
 

src/backend/core/tests/rooms/test_api_rooms_update.py

@@ -1,6 +1,7 @@
 """
 Test rooms API endpoints in the Meet core app: update.
 """
+
 import random
 
 import pytest