Fix TypeError in convert function when schema is not a class

[philippwaller]

Currently, when an incorrect type is passed for the schema parameter in the convert function, a TypeError occurs before reaching the final exception handling logic. This is especially problematic when the convert function is used recursively, as it makes debugging more challenging by failing to provide meaningful error messages.

Example of the Issue

File "/home-assistant/home-assistant-core/venv/lib/python3.13/site-packages/voluptuous_serialize/__init__.py", line 40, in convert
    pval = convert(value, custom_serializer=custom_serializer)
  File "/home-assistant/home-assistant-core/venv/lib/python3.13/site-packages/voluptuous_serialize/__init__.py", line 120, in convert
    if issubclass(schema, Enum):
       ~~~~~~~~~~^^^^^^^^^^^^^^
TypeError: issubclass() arg 1 must be a class