Fix OTIO tests

[eloisebrosseau]

Fix OTIO tests

Summarize your change.

1. Replace clip_if() with find_clips() since it has been deprecated in OTIO
2. Fix _create_transition() where post_item was undefined and if pre_item.source_range was unhashable

Describe the reason for the change.

This was preventing otio_tests::export_multi_clip_with_transition() and otio_tests::round_trip() from passing.

Describe what you have tested and on which operating system.

The tests were done on MacOS.