Allow `target` to have no distance set in `SpectralCoord`

#663 added a workaround to the fact that you can't really attach differentials to unit spherical representations. But it's basically a workaround - see https://github.com/astropy/specutils/pull/663#pullrequestreview-398475475 for a bit more on what I mean by that.

So this task is to figure out a way to make `SpectralCoord` work as intended while allowing a `target` to be in `UnitSphericalRepresentation`. This may require upstream changes in Astropy, so will have to discuss further there.

Start by reading issue #663 and the linked pull-request review, then inspect SpectralCoord's handling of UnitSphericalRepresentation. Discuss the required upstream Astropy changes before proceeding. Done means a target with no distance works as intended and the upstream coordination is resolved.

Written by the indexing model from the issue text.