Don't try to install if packages is empty

After running through pkg-map we could have no packages to install,
if so, don't attempt to run a malformed command.

Change-Id: Ia8e0aed62bcf814bf85c86b54ff0837da49ae7dd

elements/dpkg/bin/install-packages

@@ -63,4 +63,8 @@ if [ -n "$MAP_ELEMENT" ]; then
     fi
 fi
 
+if [ -z "${PKGS}" ]; then
+    echo "Not running install-packages $ACTION with empty packages list"
+else
     install_deb_packages $ACTION $PKGS
+fi