SVG Adapter erroneously assumes `media_reference` always has `target_url`

## Bug Report

### Incorrect Functionality and General Questions

(@KarthikRIyer) The SVG Adapter erroneously assumes `media_reference` always has `target_url`. Eg. `GeneratorReference` does not have `target_url`

## To Reproduce

This can be reproduced with any otio (like [test.otio.zip](https://github.com/PixarAnimationStudios/OpenTimelineIO/files/7723737/test.otio.zip) that has a `media_reference` with `"OTIO_SCHEMA": "GeneratorReference.1"`

Run `otioconvert -i test.otio -o test.svg` and you will get the output in the **Logs** section.

OS: Linux KDE Neon (Ubunt)
Python: 3.8.10
OpenTimelineIO: 0.14.1

## Expected Behavior

`otioconvert -i test.otio -o test.svg` should create `test.svg` instead of showing the error

## Logs

```
Traceback (most recent call last):
File "/home/julius/.local/bin/otioconvert", line 8, in
sys.exit(main())
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/console/otioconvert.py", line 298, in main
otio.adapters.write_to_file(
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/__init__.py", line 194, in write_to_file
return adapter.write_to_file(
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/adapter.py", line 212, in write_to_file
result = self.write_to_string(input_otio, **adapter_argument_map)
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/adapter.py", line 303, in write_to_string
return self._execute_function(
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/plugins/python_plugin.py", line 174, in _execute_function
return (getattr(self.module(), func_name)(**kwargs))
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/svg.py", line 1137, in write_to_string
return convert_otio_to_svg(input_otio, width=width, height=height)
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/svg.py", line 1127, in convert_otio_to_svg
draw_item(timeline, svg_writer, ())
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/svg.py", line 548, in draw_item
return WRITE_TYPE_MAP[type(otio_obj)](otio_obj, svg_writer, extra_data)
File "/home/julius/.local/lib/python3.8/site-packages/opentimelineio/adapters/svg.py", line 590, in _draw_timeline
item.media_reference.target_url, clip_count - 1)
AttributeError: 'opentimelineio._otio.GeneratorReference' object has no attribute 'target_url'
```

## Additional Context

Add any other context about the problem here.

Start in opentimelineio/adapters/svg.py at _draw_timeline around the failing target_url access, then reproduce the issue with the supplied GeneratorReference-based test.otio using otioconvert. The work is done when converting that input creates test.svg without the AttributeError.

Written by the indexing model from the issue text.