The issue is that the existing curves did not have an explicitly set radius.
When adding new curves with an explicit radius, the radius of the old curves was
initialized to 0 and thus becoming invisible. This patch changes it so that all
the existing curves do get a default radius. The `radius_for_write` methods is
currently only used in places that overwrite the entire array, so adding the
default there shouldn't affect anything unintentionally.
Pull Request: https://projects.blender.org/blender/blender/pulls/134445
bb715caf94