Shots processing scripts _must_ be robust to all zero inputs

[ksunden]

#9 introduced running shots processing on all 0 input to avoid problems with having to do an explicit measurement at startup, something impossible to do if e.g. the trigger is not on.

This would cause the daemon to be constantly busy and not know its channel names until the measurement concluded.

While #9 does a good job of fixing that for many workloads, it does introduce a requirement that the shots processing be able to process all 0 input.

In particular, the fs machine had shots processing script which does filtering based on chopper state of +1 or -1.
Since that is given as 0, all of the filters will return no information and thus there will be mean of empty slice and other errors relating to that filtering condition.

I mostly raise this issue as a request for documentation insofar as shots processing script requirements, rather than a request to adjust the daemon so that all shots processing scripts work (that is impossible).

So this is more a caution of an added requirement of the shots processing scripts.
The numeric output when handed all zeros is not important, the only important thing is that the names of each expected channel are returned.