Fix reference to removed null_data_source with local
Fix reference to removed null_data_source with local

A refactor that removed null_data_sources in favor of local variables
didn't update all of the locations where the old value was used; this
commit fixes up the omission.

terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory,terraform-google-modules/terraform-google-project-factory