Clearing the Parent Bone field in relations would result in something
like this:
```
add_relation(Bone Parent) - Could not find op_from
(ComponentKey(OBArmature, BONE))
add_relation(Bone Parent) - Failed, but op_to (ComponentKey(OBEmpty,
TRANSFORM)) was ok
ERROR (bke.object): /source/blender/blenkernel/intern\object.c:3330
ob_parbone: Object Empty with Bone parent: bone doesn't exist
```
Now skip creation of a depsgraph relation if the Parent Bone field is
empty (since this would be invalid anyways).
ref. T91101
Maniphest Tasks: T91101
Differential Revision: https://developer.blender.org/D12389
a45dd52cf0