fix(operator/crd): Skip serializing traffic policy field if None

We skip serialization if this is None to ensure that forwarding the
selection of the policy to Kubernetes works as expected. We stumbled
over the following unexpected behaviour:

- If ListenerClass manifests (which don't set this field) are applied
  by a client (like kubectl or helm), they are applied as expected -
  no errors.
- If the same manifests are applied by Rust code, it fails. This field
  cannot be set to "null". Serde by default serializes None to "null".

We expected there to be a null variant in the schema, which would allow
setting this field to null, but the schema only lists the two available
variants. Additionally, ad-hoc testing showed that manifests would still
be rejected after manually adjusting the schema in the CRD. This is
something which should be investigated further.

Author: Techassi

crates/stackable-operator/src/crd/listener/class/mod.rs

@@ -69,6 +69,20 @@ pub mod versioned {
         // (which currently defaults to `Cluster`). This should be the most sensible option in most cases.
         // There is the possibility Kubernetes will automatically choose `Local` if support for it on the
         // LoadBalancer has been detected.
+        //
+        // We skip serialization if this is None to ensure that forwarding the selection of the policy to
+        // Kubernetes works as expected. We stumbled over the following unexpected behaviour:
+        //
+        // - If ListenerClass manifests (which don't set this field) are applied by a client (like kubectl or helm),
+        //   they are applied as expected - no errors.
+        // - If the same manifests are applied by Rust code, it fails. This field cannot be set to "null". Serde by
+        //   default serializes None to "null".
+        //
+        // We expected there to be a null variant in the schema, which would allow setting this field to null, but
+        // the schema only lists the two available variants. Additionally, ad-hoc testing showed that manifests would
+        // still be rejected after manually adjusting the schema in the CRD. This is something which should be
+        // investigated further.
+        #[serde(skip_serializing_if = "Option::is_none")]
         pub service_external_traffic_policy: Option<core_v1alpha1::KubernetesTrafficPolicy>,
 
         /// Whether addresses should prefer using the IP address (`IP`) or the hostname (`Hostname`).