Handle bad enum values returned from the GKE API for stack_type (#7627) (#14208)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/7627.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+container: fixed a bug creating a diff adding a `stack_type` when GKE omitted `stackType` in API responses from older GKE clusters
+```

google/resource_container_cluster.go

@@ -4491,6 +4491,13 @@ func flattenIPAllocationPolicy(c *container.Cluster, d *schema.ResourceData, con
 
 	p := c.IpAllocationPolicy
 
+	// handle older clusters that return JSON null
+	// corresponding to "STACK_TYPE_UNSPECIFIED" due to GKE declining to backfill
+	// equivalent to default_if_empty
+	if p.StackType == "" {
+		p.StackType = "IPV4"
+	}
+
 	return []map[string]interface{}{
 		{
 			"cluster_ipv4_cidr_block":       p.ClusterIpv4CidrBlock,