dns-controller: default Provider when ExternalDNS is partially set

The defaulter at populate_cluster_spec.go only sets Provider=dns-controller
when Spec.ExternalDNS is nil. After #18298 added PriorityClassName, a user
(or `--override=spec.externalDns.priorityClassName=`) can leave ExternalDNS
non-nil with Provider="", which silently drops the dns-controller addon
from bootstrap-channel.yaml.

Default Provider to dns-controller whenever it is empty, regardless of
whether ExternalDNS itself is nil.

Author: claude

upup/pkg/fi/cloudup/populate_cluster_spec.go

@@ -284,9 +284,10 @@ func (c *populateClusterSpec) run(ctx context.Context, clientset simple.Clientse
 			cluster.Spec.API.PublicName = "api." + cluster.Name
 		}
 		if cluster.Spec.ExternalDNS == nil {
-			cluster.Spec.ExternalDNS = &kopsapi.ExternalDNSConfig{
-				Provider: kopsapi.ExternalDNSProviderDNSController,
-			}
+			cluster.Spec.ExternalDNS = &kopsapi.ExternalDNSConfig{}
+		}
+		if cluster.Spec.ExternalDNS.Provider == "" {
+			cluster.Spec.ExternalDNS.Provider = kopsapi.ExternalDNSProviderDNSController
 		}
 	}