Previously, some places used `curves.points_num() == 0` some other
places `curves.curves_num() == 0` to check if the geometry is empty.
Rather than having these two ways, add an `is_empty()` function
that replaces all these different checks.
Also update the curves geometry tests to use this function.
Pull Request: https://projects.blender.org/blender/blender/pulls/130168
c8211b23c7