Requesting an empty run as part of a multi_run call with add_run_id_field=False fails

[jjakob03]

In the somewhat rare use case of

• loading a run that does not return any entry (in my case, caused by usage of the selection argument)
• together with at least one other run (so multi_run is called)
• with the argument add_run_id_field=False

there is an error when trying to fetch the minimum time of an empty array in this line.

I ran into this when trying to load more than 70 runs at the same time with a quite tight selection applied, followed the warning suggesting to do run_id_as_bytes=True but then decided that I don't want to deal with the byte strings type that it produces and that I don't need the run_id at all.

To reproduce, load any list of run_ids with len(run_ids)>1 and data_type of your choice with an impossible selection, e.g.
st.get_array(run_ids,'event_info',selection='(s1_area>42)&(s1_area<42)',add_run_id_field=False).