fix: use local.zone fallback for replica region calculation

[yasra002]

Fixes potential null reference errors when var.zone is not specified by using the local.zone fallback that defaults to first available zone.

fixes: #761

╷
│ Error: Invalid function argument
│ 
│   on .terraform/modules/pg.this.cloud_psql/modules/postgresql/read_replica.tf line 40, in resource "google_sql_database_instance" "replicas":
│   40:   encryption_key_name  = (join("-", slice(split("-", lookup(each.value, "zone", var.zone)), 0, 2))) == var.region ? null : each.value.encryption_key_name
│     ├────────────────
│     │ while calling split(separator, str)
│     │ each.value is object with 16 attributes
│     │ var.zone is null
│ 
│ Invalid value for "str" parameter: argument must not be null.
╵

[imrannayer]

/gcbrun