feature: default call_name for broker.subscriber if there no any subscriber function

[Lancetnik]

Now AsyncAPI raises an error if you try to generate schema with subscriber without any handler function. We should add a default name for that case.

[Lancetnik]

I think, we can just refactor this method as the follows:

@property
def call_name(self) -> str:
    """Returns the name of the handler call."""
    if not self.calls:
        return "Subscriber"

    else:
        return to_camelcase(self.calls[0].call_name)

But, we should add the test for it to this case: https://github.com/airtai/faststream/blob/main/tests/asyncapi/base/naming.py#L15