fix: Return nil robot client when credentials are empty

When robot credentials file exists but contains empty username/password
(common in cloud-only CAPI clusters), the code would create a Robot client
with empty credentials. This client would fail with 'Unauthorized' errors
when LoadBalancer tried to attach targets.

This fix adds a check after reading credentials from file to ensure they
are not empty before creating the Robot client, matching the behavior when
reading from environment variables.

Fixes LoadBalancer target attachment failures in cloud-only clusters.

Author: don

internal/robot/client/cache/client.go

@@ -72,6 +72,10 @@ func NewCachedRobotClient(rootDir string, httpClient *http.Client, baseURL strin
 		if err != nil {
 			return nil, fmt.Errorf("%s: %w", op, err)
 		}
+		if robotUser == "" || robotPassword == "" {
+			klog.Infof("Hetzner robot is not supported because of insufficient credentials: credentials file exists but username or password is empty")
+			return nil, nil
+		}
 	}
 	c := hrobot.NewBasicAuthClientWithCustomHttpClient(robotUser, robotPassword, httpClient)
 	if baseURL != "" {