The lack of custom DIG montage is not correctly identified

Description of the problem

I have a recording MEG + 10 EEG where a .set_montage("standard_1020") fails with:

Traceback (most recent call last):

  Cell In[3], line 1
    epochs.set_montage("standard_1020")

  File <decorator-gen-25>:12 in set_montage

  File ~/git/mscheltienne/mne-python/mne/_fiff/meas_info.py:419 in set_montage
    _set_montage(info, montage, match_case, match_alias, on_missing)

  File ~/git/mscheltienne/mne-python/mne/channels/montage.py:1311 in _set_montage
    "coord_frame": info["dig"].pop()["coord_frame"],

IndexError: pop from empty list

It looks like it's due to custom_eeg_ref_dig being set to True here: https://github.com/mne-tools/mne-python/blob/7a91c470be872698dd21b5c76ab26c1bc3f4f10c/mne/channels/montage.py#L1176-L1189
Assuming the recording did have an electrode digitization for the EEG electrodes, but we actually skipped it. The ref_pos array is:

array([[1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.],
       [1., 0., 0.]])

I'll try to get some ER recordings with/without EEG channels enabled and with/without digitization to confirm on our MEG.

Start in mne/channels/montage.py at _set_montage and the custom_eeg_ref_dig handling around the referenced lines. Reproduce with a MEG+EEG recording using standard_1020, including the shown ref_pos case, and verify that missing or skipped digitization is identified without the IndexError.