Add undefined spec for successful operation

[agutierrezrodriguez]

On get operations on inexistent keys, the result is {ok, undefined} but the specs says {ok, bitstring)

[agutierrezrodriguez]

hello!! someone alive here?

[zuiderkwast]

Unfortunately, I think nobody is alive here. I have cherry-picked your spec fix to our fork: https://github.com/Nordix/eredis_cluster. We have done a lot of improvements in this fork recently.

[agutierrezrodriguez]

thanks @zuiderkwast. We'll migrate to your implementation.