Fix possible overflow of Modifier UUID
The code prior this change was re-generating modifier's session UUID
prior to copying this id from the source. This approach has a higher
risk of modifiers session UUID to overflow and start colliding with
existing modifiers.
This change makes it so that modifier copy does not re-generated the
session UUID unless it is needed.
Differential Revision: https://developer.blender.org/D14125
03ff58b67d