conversion of a MAGIC run to DL1 with a broken subrun

[jsitarek]

using magic_calib_to_dl1.py fails when processing the whole run if one of the subruns has missing reports.
an example is 20201214_M2_05093664.015_Y_CrabNebula-W0.40+035.root with no reports.
This fails when looking for Trigger tree to find the type of the run.
We could either skip the affected runs, or use the neighbouring reports: Drive is either way used for the whole run, while Trigger is not expected to change within the run (except of pathological cases).