Timeline::set_tracks(nullptr) leads to bad state

[davidbaraff]

Bug Report

Passing null to Timeline::set_tracks() leads the C++ state in a bad way: there's an implicit assumption that _tracks is always a valid pointer. If null is passed, we should create it like the constructor and just create a new empty stack, exactly as the constructor does.

To Reproduce

In Python:
import opentimelineio as otio
t = otio.schema.Timeline()
t.tracks = None
t.audio_tracks() # crashes

[ssteinbach]

closed by #961