Pre-commit hooks can force comments that aren't an empty string.
But now that you made me think about it, this process deserves scrutiny. If I add "i = 7" and commit comment "assigned 9 to i", I'm pretty sure every version control in existence will gladly accept this clear deception. That is a bug, and a dangerous one at that.
That's not a bug, it's a reason to fire a bad programmer.