ci: Add DCO placeholder check for merge queue

[chtruong814]

What does this PR do ?

Add DCO placeholder check for merge queue

We require the DCO status check on PRs to main. However, this does not work by default with merge queues because the DCO check happens on PRs only. And merge queues operate on temporary branches, so the merge queue will never succeed without the required DCO check.

So, we create a placeholder DCO check that does nothing but satisfy the required status check in merge queues. The real check will still happen on PRs.

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