make Timeline::set_tracks(null_ptr) create a new Stack to fix crash

[meshula]

By David Baraff. Supercedes #852 which had rebasing issues.

Fixes Timeline::set_tracks(nullptr) leads to bad state #851

Summarize your change.

Sets _tracks to a new Stack() if the incoming pointer is null

Reference associated tests.

Added a new test in test_tracks_set_null_tracks in tests/test_timeline.py

[codecov-commenter]

Codecov Report

Merging #961 (ed5722e) into master (2f4a3c5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #961   +/-   ##
=======================================
  Coverage   85.61%   85.61%           
=======================================
  Files         191      191           
  Lines       18204    18212    +8     
  Branches     2071     2071           
=======================================
+ Hits        15585    15593    +8     
  Misses       2088     2088           
  Partials      531      531           

Flag	Coverage Δ
unittests	85.61% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/opentimelineio/timeline.h	100.00% <ø> (ø)
src/opentimelineio/timeline.cpp	100.00% <100.00%> (ø)
tests/test_timeline.py	94.21% <100.00%> (+0.28%)	⬆️

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