Make sure $live_dev isn't an empty string

Checking whether an empty string exists as a file appears to
always be true. Check whether it's empty as well, so we don't try
to use empty strings as $live_dev

Change-Id: I9e118e32d716e9e904cd399d3b77dc1b55c9581c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1163688
Signed-off-by: Ryan Barry <rbarry@redhat.com>