fix: return early in service reconciler when lb is nil

[joey-jonko-paypal]

What does this PR do?

Fixes a nil pointer dereference panic in the service reconciler that fires when buildModel returns lb == nil — for example, when a ClusterIP-typed Service is reconciled (such as one created by Istio Gateway API via networking.istio.io/service-type: ClusterIP).

Root cause

In reconcile(), when lb == nil, the reconciler correctly enters the cleanup branch and calls cleanupLoadBalancerResources. However, on successful cleanup it falls through to reconcileLoadBalancerResources(ctx, svc, stack, lb, backendSGRequired) with lb still nil. This causes a nil pointer dereference inside reconcileLoadBalancerResources at:

lbDNS, err = lb.DNSName().Resolve(ctx) // panic: lb is nil

The panic is recovered by runtime.HandleReconcileError, so it manifests as repeated error log noise rather than a crash. Load balancer provisioning is unaffected.

Fix

Add return nil after the successful cleanup path when lb == nil, so the reconciler exits cleanly:

if lb == nil {
    // ... cleanup ...
    if err != nil {
        return ctrlerrors.NewErrorWithMetrics(...)
    }
    return nil // <-- added
}
return r.reconcileLoadBalancerResources(ctx, svc, stack, lb, backendSGRequired)

How to reproduce

1. Install Istio with Gateway API support.
2. Create a Gateway with networking.istio.io/service-type: ClusterIP (used to suppress NLB provisioning when an external ALB is managed separately via a Kubernetes Ingress resource).
3. Observe repeated panic logs from the service reconciler for the <gateway-name>-istio ClusterIP service in the controller's namespace.

Testing

Existing unit tests pass. The code path is straightforward: the only change is the addition of return nil to prevent the fall-through.

Closes #4724

---

Release note:

Fix nil pointer dereference panic in service reconciler when reconciling a ClusterIP service (e.g. created by Istio Gateway API with networking.istio.io/service-type: ClusterIP) where buildModel returns lb == nil.

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[wweiwei-li]

Do you want to fix EasyCLA ? Else, I can submit a change for this.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.13%. Comparing base (22cdb42) to head (b299b3d).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
controllers/service/service_controller.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4725      +/-   ##
==========================================
+ Coverage   56.09%   56.13%   +0.03%     
==========================================
  Files         388      388              
  Lines       30932    30948      +16     
==========================================
+ Hits        17352    17373      +21     
+ Misses      12566    12564       -2     
+ Partials     1014     1011       -3     

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: joey-jonko-paypal, sandeep-rajan-glean
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