Behavior for Controller.get_exit_policy() for tor client

In #25423 (closed) it was noticed that...

[...] the client-only configuration ALWAYS returns this: {{{

GETINFO exit-policy/full 551 router_get_my_routerinfo returned NULL }}}

#25852 (moved) tracks implementing defined behavior in tor and the control spec to return 551 in this scenario.

This ticket tracks any necessary changes to stem.control for Controller.get_exit_policy() to return the appropriate ExitPolicy (perhaps None) for tor clients.