Fix slack notification for nightly tests (#3504)

Since nightly tests are run on a schedule and not triggered from a commit or PR, we do not have `github.event.pull_request.html_url` or `github.event.head_commit.url` available.

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#### Does this PR need a docs update or release note?

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- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

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- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
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- [ ] 🤖 Supportability/Tests
- [x] 💻 CI/Deployment
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#### Test Plan

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- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

.github/workflows/nightly_test.yml

@@ -174,7 +174,7 @@ jobs:
                   "type": "section",
                   "text": {
                     "type": "mrkdwn",
-                    "text": "[FAILED] Nightly Checks :: <${{  steps.sha-info.outputs.RUN_URL }}|[Logs]> <${{ github.event.pull_request.html_url || github.event.head_commit.url }}|[Base]>\nCommit: <${{  steps.sha-info.outputs.COMMIT_URL }}|${{  steps.sha-info.outputs.SHA }}>"
+                    "text": "[FAILED] Nightly Checks :: <${{  steps.sha-info.outputs.RUN_URL }}|[Logs]> <${{ steps.sha-info.outputs.COMMIT_URL }}|[Base]>\nCommit: <${{  steps.sha-info.outputs.COMMIT_URL }}|${{  steps.sha-info.outputs.SHA }}>"
                   }
                 }
               ]