Project Management Automation: Commit-to-master fails addMilestone task

[aduth]

Related: #20058

Describe the bug

Direct commits to the master branch will result in a failing action for the addMilestone task.

See: https://github.com/WordPress/gutenberg/runs/436475328

main: Received event = 'push', action = 'undefined'
main: Starting task addMilestone
##[error]main: Task addMilestone failed with error: TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))
main: All done!
##[error]Node run failed with exit code 1

While commits to master are discouraged and should be rare occurrences, the action should be able to handle this gracefully.

Based on the error message, I suspect that the issue is that String#match will return null when there is no match:

gutenberg/packages/project-management-automation/lib/add-milestone.js

Line 44 in 38f4914

const [ , prNumber ] = payload.commits[ 0 ].message.match( /\(#(\d+)\)$/m );

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match#Return_value

Before deconstructing the array members, this should check that the result would be non-null.