pacman.py: Fix incorrect return in pkg.latest_version
pacman.py: Fix incorrect return in pkg.latest_version

This was initially broken in 75fd92f. The documented behavior of this
function is to return an empty string for the package when the package
is up-to-date. Changing that behavior negatively affects the pkg.latest
state, which relies on the documented behavior.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt