Raise UserError when tool system_prompt returns a None

[cmungall]

Previously

@agent.system_prompt
async def func():
    return None

would raise an error at time of model calling

This PR checks defensively checks what is being returned, and will raise a hopefully more informative UserError. Note this all requires a type check override, since returning None is forbidden. However, if we treat user code as input then it's possible to violate at runtime.

Also include a unit test, and for an additional ultra-edgy case of returning a non-string from system_prompt

Fixes #1057

[cmungall]

Either intentionally or accidentally returning a number seems quite unlikely but I decided to cast to str and add a test.