[core] Avoid failure of loading back the user starred app
Traceback (most recent call last):
File "/hue/desktop/core/src/desktop/models.py", line 1792, in get_main_quick_action
user_default_app = json.loads(user_preference)
File "/usr/lib/python3.8/json/__init__.py", line 341, in loads
raise TypeError(f'the JSON object must be str, bytes or bytearray, '
TypeError: the JSON object must be str, bytes or bytearray, not dict