ScalerCH.select_channels() accepts channel named ""

[prjemian]

Gets a bit complicated here. Ultimately, though, ophyd.scaler.ScalerCH.select_channels([""]) is an allowed possibility and that should not be allowed.

ScalerCH differs from EpicsSignal by exposing (and using) the EPICS scaler channel names to ophyd users.

In ophyd.scaler.ScalerCH.match_names(), the text names configured in the EPICS scaler PV .NMn fields are copied to the local scaler channel name. This is great since we can then focus on describing/reading/recording only the named channels in EPICS, assuming that unnamed channels are not in use and otherwise not worthy of our attention.

In select_channels(), the name_map thus will have a key that corresponds to the last unnamed channel. Since this channel is in the name_map, it is also a permissible selection candidate for the chan_names Iterable. The dictionary expression, nicely compact, is too tricky in this case and misses this typical scenario.

[prjemian]

Remove this line