flagger crashes if you define SLACK_URL env var and there's nothing in slack.url parameter

[haisumb]

I created this HelmRelease:

apiVersion: helm.fluxcd.io/v1
kind: HelmRelease
metadata:
  name: flagger
  namespace: istio-system
spec:
  releaseName: flagger
  chart:
    repository: https://flagger.app
    name: flagger
    version: 0.24.0
  values:
    meshProvider: istio
    metricsServer: http://prometheus:9090
    env:
    - name: SLACK_URL
      valueFrom:
       secretKeyRef:
         name: slack
         key: url
    slack:
      # This will crash flagger
      # url: 
      url: "must provide something here. otherwise flagger crashes"
      channel: haisum
      user: flagger

If I skip url from above, slack integration doesn't work. We get following logs in flux when it tries to do canary:

{"level":"info","ts":"2020-03-10T17:46:50.547Z","caller":"flagger/main.go:110","msg":"Starting flagger version 1.0.0-rc.1 revision e59acc7bae6b2d1c9a3e97abb2dcadc9f0985c2d mesh provider istio"}
{"level":"info","ts":"2020-03-10T17:46:50.598Z","caller":"flagger/main.go:381","msg":"Connected to Kubernetes API v1.14.8"}
{"level":"info","ts":"2020-03-10T17:46:50.598Z","caller":"flagger/main.go:235","msg":"Waiting for canary informer cache to sync"}
{"level":"info","ts":"2020-03-10T17:46:50.698Z","caller":"flagger/main.go:242","msg":"Waiting for metric template informer cache to sync"}
{"level":"info","ts":"2020-03-10T17:46:50.798Z","caller":"flagger/main.go:249","msg":"Waiting for alert provider informer cache to sync"}
{"level":"info","ts":"2020-03-10T17:46:50.901Z","caller":"flagger/main.go:158","msg":"Connected to metrics server http://prometheus:9090"}
{"level":"error","ts":"2020-03-10T17:46:50.901Z","caller":"flagger/main.go:323","msg":"Notifier empty Slack channel","stacktrace":"main.initNotifier\n\t/home/circleci/build/cmd/flagger/main.go:323\nmain.main\n\t/home/circleci/build/cmd/flagger/main.go:164\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:203"}
{"level":"info","ts":"2020-03-10T17:46:50.901Z","caller":"controller/controller.go:153","msg":"Starting operator"}
{"level":"info","ts":"2020-03-10T17:46:50.902Z","caller":"controller/controller.go:162","msg":"Started operator workers"}
{"level":"info","ts":"2020-03-10T17:46:50.902Z","caller":"server/server.go:29","msg":"Starting HTTP server on port 8080"}
{"level":"info","ts":"2020-03-10T17:46:50.907Z","caller":"controller/controller.go:239","msg":"Synced default/helloworld"}
{"level":"info","ts":"2020-03-10T17:47:00.973Z","caller":"controller/events.go:15","msg":"New revision detected! Scaling up helloworld.default","canary":"helloworld.default"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x145cf78]

goroutine 76 [running]:
github.com/weaveworks/flagger/pkg/notifier.(*Slack).Post(0x0, 0xc000267960, 0xa, 0xc000267979, 0x7, 0x180e2f5, 0x30, 0xc00013f440, 0x4, 0x6, ...)
	/home/circleci/build/pkg/notifier/slack.go:66 +0x68
github.com/weaveworks/flagger/pkg/controller.(*Controller).alert(0xc0002037a0, 0xc000063c00, 0x180e2f5, 0x30, 0xc0005e5b01, 0x17b80f3, 0x4)
	/home/circleci/build/pkg/controller/events.go:66 +0x1519
github.com/weaveworks/flagger/pkg/controller.(*Controller).checkCanaryStatus(0xc0002037a0, 0xc00048ea80, 0x1b3e520, 0xc00003a640, 0x1, 0x0)
	/home/circleci/build/pkg/controller/scheduler.go:629 +0x72b
github.com/weaveworks/flagger/pkg/controller.(*Controller).advanceCanary(0xc0002037a0, 0xc00057e9e0, 0xa, 0xc00057e9f9, 0x7, 0x445e00)
	/home/circleci/build/pkg/controller/scheduler.go:182 +0x9f9
github.com/weaveworks/flagger/pkg/controller.CanaryJob.Start.func1(0xc00057e9e0, 0xa, 0xc00057e9f9, 0x7, 0x0, 0xc000472700, 0xc0001003c0, 0xc0002ae6e0, 0x37e11d600)
	/home/circleci/build/pkg/controller/job.go:20 +0x7b
created by github.com/weaveworks/flagger/pkg/controller.CanaryJob.Start
	/home/circleci/build/pkg/controller/job.go:18 +0x70