Don't run oojs-ui-phplint for changes without php files

The job itself already filters this:
> ../git-changed-in-head php php5 inc phtml module install

But we still trigger the job (running against 0 files).
Optimise this similar to other lint jobs.

Change-Id: Ic7b46d87bca67723468c0d21031544e9c2d56af0