(maint) Don't try to ship debs if there are none

Previously the ship_debs task would fail hard if the pkg/deb directory didn't
exist, which makes sense. This commit updates the ship_debs task to not bother
rsyncing debs if there is no pkg/deb directory to rsync, as is the case for
pe-razor-server currently.
Merge pull request #243 from haus/fix_ship_debs

(maint) Don't try to ship debs if there are none