When e.g. executing `drawing.strokes[0].softness = 3`, the API would
always create a new attribute `softness` even if that attribute existed
already.
The issue was that the code was using the `.get(value, fallback)` syntax
but the `fallback` expression is always evaluated by python.
The fix removes the use of the `fallback` and uses a simple `if/else` to
check if the attribute doesn't exist yet and only then create it.
Pull Request: https://projects.blender.org/blender/blender/pulls/129044
39675bb0fb