[PRO-155] Create empty commit on pr if the HEAD_SHA is from a user outside the org (forked branch)

[NicHaley]

Review comments do not currently work if the HEAD_SHA is from a forked branch. This is because the GH review comment API requires a commit_id, which in this case belong to a user outside the org. The method will thus give a 401.

A possible solution would be to create an empty commit if the HEAD_SHA is from outside the org. Then, pass the empty commit SHA to the review comment command.

Ideally, this functionality should be attached to a flag so that the org can determine if they want to opt into this functionality.

_{From SyncLinear.com | PRO-155}

[NicHaley]

Another solution in the mean time would be to put an action like this in front: https://github.com/marketplace/actions/create-empty-commit