set -u doesn't permit bash arrays to test -n

Although set -u allows empty WHITELIST=() list to be set, you can't test
-n "$WHITELIST".  Looks like the intention of the commit that caused
this issue (Change-ID Iad2d490c86dceab148ea9ab08f457c49a5d5352e) is to
set a policy to prevent future use of bugs due to unbound variables.

Thanks StevenK for pairing with me and helping me hone my bash skills.

Change-Id: I2349c990ba53b0aaecd5119812582c7c848f1e4e
Closes-Bug: #1319987

elements/yum/bin/install-packages

@@ -43,7 +43,7 @@ do
         echo "The package $i is not available and will not be installed"
     fi
 done
-if [ -n "$WHITELIST" ]; then
+if [ ${#WHITELIST[@]} -eq 0 ]; then
     if [ -f /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release ]; then
         rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
     fi