sdss-v mwmVisit data loaders broken

The SDSS-V data loader for `mwmVisit` fails to load `mwmVisit` files that only have BOSS-extension data in them. It loads fine for `mwmVisit` files with only APOGEE-extension data. I think the issue is during metadata collection, it looks for info specific to certain extensions but not all. e.g. see https://github.com/astropy/specutils/blob/67bea40efb4e4a01672241123db6c225b3954d14/specutils/io/default_loaders/sdss_v.py#L589

We could either remove the metadata entirely as it's not strictly necessary to load the spectrum, or adjust this code. This is a blocker for https://github.com/sdss/sdss_solara/issues/9

Either I or relevant folks can fix, just creating an issue for tracking.

Start in specutils/io/default_loaders/sdss_v.py around line 589 and compare metadata collection for BOSS-only and APOGEE-only mwmVisit files. Confirm the failure with a BOSS-extension-only file, then determine whether metadata can be removed or made extension-independent. Done means BOSS-only mwmVisit files load successfully without breaking APOGEE-only files.

Written by the indexing model from the issue text.