event send to slack fail

[13567436138]

Describe the bug

A clear and concise description of what the bug is. Please provide the Canary definition and Flagger logs.

To Reproduce

      - name: "send to Slack"
        type: event
        url: https://hooks.slack.com/services/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
        metadata:
          environment: "test"
          cluster: "flagger-test" 
```
<!--
Steps to reproduce the behaviour
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{"level":"error","ts":"2021-09-05T23:54:24.469Z","caller":"controller/events.go:57","msg":"error sending event to webhook: no_text","canary":"bookinfo.istio","stacktrace":"github.com/fluxcd/flagger/pkg/controller.(*Controller).sendEventToWebhook\n\t/workspace/pkg/controller/events.go:57\ngithub.com/fluxcd/flagger/pkg/controller.(*Controller).recordEventWarningf\n\t/workspace/pkg/controller/events.go:47\ngithub.com/fluxcd/flagger/pkg/controller.(*Controller).runConfirmRolloutHooks\n\t/workspace/pkg/controller/scheduler_hooks.go:53\ngithub.com/fluxcd/flagger/pkg/controller.(*Controller).advanceCanary\n\t/workspace/pkg/controller/scheduler.go:244\ngithub.com/fluxcd/flagger/pkg/controller.CanaryJob.Start.func1\n\t/workspace/pkg/controller/job.go:39"}
### Expected behavior

A clear and concise description of what you expected to happen.

### Additional context

- Flagger version:
- Kubernetes version:   
- Service Mesh provider:
- Ingress provider: