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).

Change-Id: I315f97fe619c25b36cf47ed1b7a65936b753312e

elements/yum/bin/install-packages

@@ -77,7 +77,10 @@ if [ -n "$WHITELIST" ]; then
     else
         PKGS=$(map-packages $WHITELIST)
     fi
-    echo "Installing $PKGS"
+    if [ -z "${PKGS}" ]; then
+        echo "Not running install-packages $ACTION with empty packages list"
+    else
+        echo "Running install-packages ${ACTION}. Package list: $PKGS"
         yum -y $ACTION $EXTRA_ARGS $PKGS
         for pkg in "$@"; do
             if [ "$pkg" = "python-pip" ] ; then
@@ -85,3 +88,4 @@ if [ -n "$WHITELIST" ]; then
             fi
         done
     fi
+fi