find_builtin_server_type should return Optional server type

This should not raise an error: https://github.com/idom-team/idom/blob/8a99b452fd877bea78b9718c1e32e8855c768166/src/idom/server/prefab.py#L16

It should just return `None` if no server implementations are installed. An error should then be raised at runtime if no server is explicitly given and no default exists.