Prevent empty notifications when prepare fails.

[desrosj]

Because of the failure() check, the failure job runs even when the message's payload is not properly prepared.

This adds an additional check to guard against scenarios where the payload output is empty.

This was discovered in this workflow run. The raw logs are being attached so that they persist after the 90 day retention period for logs passes.

• prepare log.txt
• failure log.txt

Trac ticket: Core-64893.

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Tool(s): Claude
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[github-actions]

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Props desrosj.

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[Copilot]

Pull request overview

Updates the reusable Slack notification workflow to avoid posting empty Slack notifications when the prepare job does not produce a payload output (e.g., when prepare fails).

Changes:

• Gate the failure notification job on needs.prepare.outputs.payload being non-empty.
• Gate the cancelled notification job on needs.prepare.outputs.payload being non-empty.

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