Fix a reversed error check in impl.Export{}.WeakNil.
Check to see if we have a type for the weak field on marshal/size.
Treat a typed nil valued in XXX_Weak as not indicating presence for
the field.
Change-Id: Id667ac7eb4f53236be9e181017082bd8cd21d115
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/198717
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
