fix(variables): fail gracefully if `sources` index does not exist

[eeaton]

add logic to fail gracefully if the map reference egress_policies.value["from"]["sources"] or ingress_policies.value["from"]["sources"] fails because the "sources" index does not exist. Both the from and sources blocks are optional in provider docs.

• https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/access_context_manager_service_perimeter_egress_policy
• https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/access_context_manager_service_perimeter_ingress_policy

Issue identified in: https://github.com/terraform-google-modules/terraform-example-foundation/pull/1251#issuecomment-2128298785

[daniel-cit]

/gcbrun