nir/propagate_invariant: Don't add NULL vars to the hash table

Fixes: 8410cf66 "nir/propagate_invariant: Skip unknown vars"
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit d96878a6)