added default_value fallback to serializable_field property

[timlehr]

Link the Issue(s) this Pull Request is related to.

Fixes #1657

Summarize your change.

Simple addition to the code to make sure access to a field that hasn't been initialized yet doesn't fail and returns a fallback default value instead.

[codecov-commenter]

Codecov Report

Merging #1658 (1b18f60) into main (45c6cfd) will decrease coverage by 0.01%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1658      +/-   ##
==========================================
- Coverage   79.91%   79.90%   -0.01%     
==========================================
  Files         197      197              
  Lines       21731    21731              
  Branches     4339     4339              
==========================================
- Hits        17367    17365       -2     
- Misses       2213     2214       +1     
- Partials     2151     2152       +1     

Flag	Coverage Δ
py-unittests	79.90% <100.00%> (-0.01%)	⬇️

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Files Changed	Coverage Δ
.../py-opentimelineio/opentimelineio/core/__init__.py	83.33% <100.00%> (ø)

... and 1 file with indirect coverage changes

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