fix: Only create MetalLB configuration when necessary (#791)

**What problem does this PR solve?**:
The ServiceLoadBalancer configuration is optional; when it is not set,
we should not configure the provider.

Thanks to @dkoshkin for finding this bug.

The e2e tests in #788 did not catch this, because they do define
configuration.

**Which issue(s) this PR fixes**:
Fixes #

**How Has This Been Tested?**:
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Author: dlipovetsky

pkg/handlers/generic/lifecycle/serviceloadbalancer/metallb/configuration.go

@@ -26,6 +26,10 @@ type configurationInput struct {
 }
 
 func configurationObjects(input *configurationInput) ([]unstructured.Unstructured, error) {
+	if len(input.addressRanges) == 0 {
+		return nil, fmt.Errorf("must define one or more addressRanges")
+	}
+
 	ipAddressPool := unstructured.Unstructured{}
 	ipAddressPool.SetGroupVersionKind(groupVersionKind("IPAddressPool"))
 	ipAddressPool.SetName(input.name)

pkg/handlers/generic/lifecycle/serviceloadbalancer/metallb/handler.go

@@ -178,6 +178,11 @@ func (n *MetalLB) Apply(
 		return fmt.Errorf("failed to wait for MetalLB to deploy: %w", err)
 	}
 
+	if slb.Configuration == nil {
+		// Nothing more to do.
+		return nil
+	}
+
 	log.Info(
 		fmt.Sprintf("Applying MetalLB configuration to cluster %s",
 			ctrlclient.ObjectKeyFromObject(cluster),