relative_path should not be auto-generated for location types other than dataregistry and dummy

[JoanneBogart]

There is this code in register:

 # If `relative_path` not passed, automatically generate it
    if kwargs_dict["relative_path"] is None:
        kwargs_dict["relative_path"] = _relpath_from_name(
            name, kwargs_dict["version_string"], kwargs_dict["old_location"]
        )

But for location types external and meta_only there is no meaningful relative path. I would say the most natural value in that case is None. Is there any reason not to modify the above to

# If `relative_path` not passed, automatically generate it
      if kwargs_dict["relative_path"] is None and kwargs_dict["location_type"] in ["dataregistry", "dummy"]:

and so forth

[JoanneBogart]

It turns out there is: currently we have a non-null constraint for relative_path.
Probably we should eliminate that constraint, but if so, I'd like to do it in-place for the schemas lsst_desc_production, lsst_desc_working and the corresponding tutorial schemas.

The other alternative would be to use some other sentinal value for meta_only and external. Currently for meta_only location type I'm also setting relative_path to meta_only, but that's probably not a good idea since it collides with a legitimate relative path. A better choice might be something like ?None? That is, include a character or characters not normally allowed for relative_path.

This is what I've done for now. That is, for all the catalog entries with location type meta_only I've changed relative_path to ?None? This will require a small change to the code to not do validity-checking on the relative_path field in case location type is meta_only or external (or, maybe better, set the special values itself).