fix: handle non-resuscitation recordings in read_zollcsv

[sorlob]

When a recording has no CPR activity, CprDepth and CprFrequency columns are all zeros. After filtering out zeros, timestamps1 is empty, causing IndexError on timestamps1[-1] and subsequent KeyError on CompDisp/CompRate accesses that assumed CPR data was always present.
#276

Additionally omits import of vast zero arrays.

Example File for ZOLL R Series containing ECG an PPG:
20260331124955_00147042.zip

[behackl]

This reviewer fix adds a synthetic regression test for ZOLL CSV imports with
no CPR activity and simplifies read_zollcsv to treat all-zero channels as
absent instead of propagating empty placeholder DataFrames. The loader now
skips zero-only CPR and waveform channels, guards all later optional accesses
to CompDisp and CompRate, and avoids emitting phantom metadata rows for
omitted channels. This makes the behavior more consistent, fixes the
non-resuscitation crash described above, and ensures end-to-end loading
still works when a recording contains ECG/capno/SpO₂ data but no
chest-compression data.

[behackl]

Also adds a regression test. If you really need the empty channels, we can change this fix in a follow-up PR; for now I'd like to get this merged. Other PRs in this chain to follow.