I found yet another and more elegant solution (in my opinion) to eliminate empty commit messages.
In Gerrit project options you may set the following options to avoid empty workitems if git is able to do merge with rebase automatically (there are no conflicts which need human intervention):
- Projects -> General -> Submit Type -> Rebase if Necessary
- Projects -> General -> Automatically resolve conflicts -> TRUE
In fact, with such strategy, gerrit will inform that automatic merge is not possible and will do nothing. On the other hand, if rebase+automatic merge is successful, there will be just one for/master->master workitem.