Don't do work if no triggers

[spxiwh]

I have noticed that pycbc_live makes many calls to the expensive logsignalrate function with no triggers (or at least it does in the first minutes). These calls at least can be made much quicker by bypassing them early on.

.... Not really sure this will help much once pycbc_live has a full buffer of background triggers, but I don't see any reason not to merge this, while I figure out how to meaningfully profile with a full buffer without having to wait 10s of hours (and hope that we are always analysing 3 detectors of mock data).