Handle missing apt lists

On a newly imaged system, /var/lib/apt/lists may not exist. Check for
this case before comparing dates to the sources.

cdist/conf/type/__apt_update_index/gencode-remote

@@ -34,7 +34,7 @@ apt_opts="-o Acquire::AllowReleaseInfoChange::Suite=true -o Acquire::AllowReleas
 # run 'apt-get update' if anything in /etc/apt is newer then /var/lib/apt/lists
 #   it will be run a second time on error as a redundancy messure to success
 cat << DONE
-if find /etc/apt -mindepth 1 -cnewer /var/lib/apt/lists | grep . > /dev/null; then
+if [ ! -d /var/lib/apt/lists ] || { find /etc/apt -mindepth 1 -cnewer /var/lib/apt/lists | grep . > /dev/null; }; then
    apt-get $apt_opts update || apt-get $apt_opts update
 fi
 DONE