HHVM: set an empty string as the value for hhvm.pid_file

If hhvm.pid_file is unset, HHVM uses a default PID file path. To convince HHVM
not to try and manage its PID file at all, we need to set an empty string.

Change-Id: If7e8af3df3e87b169f3a0ed5b16ec0b57fc4b9d7