If a bone was specified (but now missing), the driver would fallback to
using the object as a target (which can lead to unintended behavior).
So now check if a bone is specified, if it is missing, mark the driver
invalid. If no bone is specified at all, then use the armature object as
the target.
NOTE: `DTAR_FLAG_INVALID` is not granular enough to distinguish the
object and bone targets, so both will be marked in red in the UI (there
is already comments about it in code). If necessary, we could introduce
an additional DTAR_FLAG_BONE_INVALID and use that in a couple of places.
Pull Request: https://projects.blender.org/blender/blender/pulls/119533
5ba581bd0a