GetCurrentTimeslot(): handle there being no current timeslot

[markspolakovs]

I'd quite like to just return nil, but that'd involve changing the return type to *Timeslot which is a breaking change >:(

[coveralls]

Coverage decreased (-0.05%) to 20.0% when pulling d9c66c6 on marks/gettimeslot-none into 2b440aa on master.

[Brookke]

Should probably include a test.

How much is actually using this part of myradio-go, just 2016-site right? If so I expect you could do the breaking change wouldn't be too much work if you did want to go down that route?