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Transform: Use parent bone orientation if the bone has not size

As shown in the T68805, non-sized bones (such as the resulting extruded
bone) have no direction or orientation.

This can be bad for operators like `extrude_move` since the user might
want the resulting bone to be aligned with the bone that originated it.

The solution here is to get the parent bone orientation in the
transform operator if the bone has no size.

Differential Revision: https://developer.blender.org/D6486