Always write out output file even when no events are selected

Currently productions with low selection rates often have issues when files have a small number of events and end up selecting nothing.

It would be nicer if FunTuple could always produce it's TTrees even if they contain no events. This would allow LHCbDIRAC to treat the jobs as successful and continue to merge the output files instead of marking the job as failed for not producing output.

(This might already be the case for Run 3 DaVinci but I just had a bunch of painful Run 1 productions to deal with for this issue.)