improper handling of tracker.get_slot("amount-of-money") in validate_credit_card method

[BGLouie]

in validate_credit_card method, the amount-of-money slot may contain a number (e.g 5) or a string (e.g minimum) which is then evaluated in if amount and amount.lower() in balance_types leading to AttributeError: 'int' object has no attribute 'lower' if amount is a number.
Tested with:
Python 3.8.18
rasa 3.1.0
rasa-sdk 3.1.3