bootstrap:autointf: gen_on_field: fix: move optional fields only if set
bootstrap:autointf: gen_on_field: fix: move optional fields only if set

Otherwise, moving a nil optional field would generate (for
non-`Trivial_move):

	self.f.Nonnil = 1
	self.f.X.Move other.f.X

We need to do this, but conditionally.

nanocritical/n,nanocritical/n,nanocritical/n,nanocritical/n