Allow merging PullRequests with commitMessage unspecified

[pratikmallya]

If the commitMessage is unspecified, merging a PR with squash merge commit results in commit with concatenated text from all source commits. example.

This field should be omitted when the commit message is empty; in that case on squash merge this behavior would be triggered.

I do concede it would be a small change in behavior; if a current user is expecting the squash merge with empty commit message to really have an empty commit message. Not sure how to differentiate between wanting to force an empty commit message and wanting the desired behavior.