Remove assumption that clips have a media reference of type `ExternalReference`

Some of the adapters make the assumption that clips are pointing at media references with ExternalReferences implicitly by assuming that media_references have a `target_url`. We should probably try and remove this assumption.

Some of the adapters:
- [x] AAF (writer)
- [ ] FCP XML (Almost!) https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/1dbe0fcf945d2b4925c3e61886024ce4ca49c56e/src/py-opentimelineio/opentimelineio/adapters/fcp_xml.py#L1690
- [ ] CMX EDL (https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/1dbe0fcf945d2b4925c3e61886024ce4ca49c56e/src/py-opentimelineio/opentimelineio/adapters/cmx_3600.py#L432)
- [ ] SVG: https://github.com/PixarAnimationStudios/OpenTimelineIO/blob/1dbe0fcf945d2b4925c3e61886024ce4ca49c56e/src/py-opentimelineio/opentimelineio/adapters/svg.py#L590

Start by reviewing the unchecked adapter locations in fcp_xml.py, cmx_3600.py, and svg.py, especially the referenced lines where media_references are assumed to have target_url. Compare them with the completed AAF writer item and trace how clips with other media-reference types are handled. Done means the listed unchecked adapters no longer rely implicitly on ExternalReference or target_url.

Written by the indexing model from the issue text.