[Breaking change] Remove support for passing a None value as the name argument

[JeanChristopheMorinPerso]

Remove support for passing a None value as the name argument to Marker, Track, Stack and MissingReference constructors

Follow up to #1446.

As discussed in the last TSC meeting, it has been agreed that we shuld keep the class constructors consistent, which mean that we should drop the ability to pass None values for the name argument of the Marker, Track, Stack and MissingReference class constructors.

Passing None is bad in this case because it's inconsistent with the other constructors, which can be seen as not user friendly.

The name parameter is always defined as a kwarg with a default value of '' (empty string). This means that it's still possible to ignore the name parameter entirely when instantiating a class. The only difference now is that the type will be explicitly exposed (and discoverable) and if something else than a string is passed as an argument, it will cause an exception like this:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: __init__(): incompatible constructor arguments. The following argument types are supported:
    1. opentimelineio._otio.Marker(name: str = '', marked_range: opentimelineio._opentime.TimeRange = otio.opentime.TimeRange(start_time=otio.opentime.RationalTime(value=0, rate=1), duration=otio.opentime.RationalTime(value=0, rate=1)), color: str = 'RED', metadata: object = None)

Invoked with: kwargs: name=None

This is considered a breaking change and should be clearly marked as such in the changelog.

[codecov-commenter]

Codecov Report

Merging #1453 (788a193) into main (c2507e1) will increase coverage by 0.00%.
The diff coverage is 33.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1453   +/-   ##
=======================================
  Coverage   78.88%   78.89%           
=======================================
  Files         200      200           
  Lines       21728    21724    -4     
  Branches     4406     4403    -3     
=======================================
- Hits        17141    17140    -1     
  Misses       2354     2354           
+ Partials     2233     2230    -3     

Flag	Coverage Δ
py-unittests	78.89% <33.33%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
...entimelineio-bindings/otio_serializableObjects.cpp	26.59% <0.00%> (+0.01%)	⬆️
...-opentimelineio/opentimelineio/adapters/fcp_xml.py	88.40% <100.00%> (ø)

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