sdss-v mwmVisit data loaders broken

[havok2063]

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.