Add new DiffSuppress for dns managed zone visibility (#3461)

Signed-off-by: Modular Magician <magic-modules@google.com>

google/resource_dns_managed_zone.go

@@ -335,6 +335,9 @@ func flattenDnsManagedZoneLabels(v interface{}, d *schema.ResourceData) interfac
 }
 
 func flattenDnsManagedZoneVisibility(v interface{}, d *schema.ResourceData) interface{} {
+	if v == nil || v.(string) == "" {
+		return "public"
+	}
 	return v
 }
 

google/resource_dns_managed_zone_test.go

@@ -77,6 +77,8 @@ resource "google_dns_managed_zone" "foobar" {
 	labels = {
 		foo = "bar"
 	}
+
+	visibility = "public"
 }`, suffix, suffix, description)
 }