isl_union_*_transform: allow missing transformation function
isl_union_*_transform: allow missing transformation function

This allows isl_union_*_transform to be used in a mode
that only filters some of the base expressions.
This will be used to implement isl_union_*_subtract_domain_space
in an upcoming commit.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl