The interpolation tool in edit mode would create an empty keyframe if nothing in the
`from_drawing` and `to_drawing` was selected. This is not how the tool behaved in GPv2
so this is unexpected.
The fix checks that if `only_selected` is used, there also is a selection in both of the
drawings in the pair, and otherwise fallback to interpolating all the strokes.
This is consistent with how the tool worked in 4.2.
Pull Request: https://projects.blender.org/blender/blender/pulls/129206
078f16a8d5