Avoid to install a blank list of packages

There are some situations where the list of packages to install
in a image is blank (packages blacklisted) on opensuse based
distributions.

Change-Id: Ib720473fa63671ed974d41a86fe10a9b10720f5d

elements/opensuse/bin/install-packages

@@ -78,5 +78,9 @@ if [ -n "$WHITELIST" ]; then
     else
         PKGS=$(map-packages $WHITELIST)
     fi
+    if [ -z "${PKGS}" ]; then
+        echo 'No packages need to be installed'
+    else
         run_zypper install $EXTRA_ARGS $PKGS
     fi
+fi