Handle empty extra_package gracefully

On Ubuntu 12.04 LTS it's possible:

    %x(apt-cache search linux-image-extra-`uname -r | grep --only-matching -e [0-9]\.[0-9]\.[0-9]-[0-9]*` | cut -d " " -f 1).strip

..returns an empty string.  This causes a package[] error.  Added a simple empty string check to handle this error condition.