Fix an incorrect assumption in get_instruments_in_ranking_statistic()
The function currently assumes that only detectors with a valid chi^2 field contribute to the ranking of an event. This is not true for PyCBC Live in O3, which can have empty chi^2 values even though all detectors always contribute to the ranking.
This patch fixes the get_instruments_in_ranking_statistic()
function to always return all detectors for events from
PyCBC Live.
We are testing a change to PyCBC Live which will make this problem important. This is necessary for PyCBC Live to continue working well after that change.