fix: Router advertised-route-priority undefined behavior

[modular-magician]

Bug: b/356353797
Issue: advertised-route-priority is an optional field and if not present, the value in the GCP is considered to be 100.
However, TF would set advertised-route-priority to be 0 even if the user has not explicitly set the value to be 0 when there is an update to the resource.

Repro steps:

1. Create a Router peer resource without advertised route priority value set.
   resource "google_compute_router_peer" "foobar" {
   name = "tf-my-router"
   router = google_compute_router.foobar.name
   region = google_compute_router.foobar.region
   peer_asn = 65515
   interface = google_compute_router_interface.foobar.name
   advertise_mode = "DEFAULT"
   }

Query gcloud to check the value of advertised_route_priority, it will be empty
gcloud compute routers describe {router-name}

2. Update the router peer resource (ex: enableIpv6 = true) without advertised route priority. However, TF would add advertised route priority = 0 in the update api call.
   resource "google_compute_router_peer" "foobar" {
   name = "tf-my-router"
   router = google_compute_router.foobar.name
   region = google_compute_router.foobar.region
   peer_asn = 65515
   interface = google_compute_router_interface.foobar.name
   advertise_mode = "DEFAULT"
   enable_ipv6 = true
   }

Query gcloud to check the value of advertised_route_priority, it will be 0 even though it is not set by the user.
gcloud compute routers describe {router-name}

Release Note Template for Downstream PRs (will be copied)

compute: fixed a bug where `advertised_route_priority` was accidentally set to 0 during updates in 'google_compute_router_peer'

Derived from GoogleCloudPlatform/magic-modules#11613