BUG: MNE-BIDS coord frame regression

[larsoner]

Could it be that this is also the reason that the mne-bids test pipeline is currently failing?

https://github.com/mne-tools/mne-bids/actions/runs/13351688224/job/37289052292

FAILED mne_bids/tests/test_read.py::test_get_head_mri_trans - ValueError: Coordinate frame not supported: None
================== 1 failed, 288 passed in 264.35s (0:04:24) ===================

Originally posted by @sappelhoff in #13112 (comment)

[larsoner]

@sappelhoff I think that's more likely caused by #13083. I refactored some stuff and thought some code was equivalent but it maybe wasn't

[larsoner]

Fixed now!