If the /proc/sys/net/netfilter/nc_conntrack_* files are not present,
this exporter was outputting a blank value, which is invalid. These
files will not be present when using an image that doesn't include the
iptables and firewall packages (eg a minimal access-point type image).

This updates the collector to only output the metrics if the
corresponding /proc files are present.

Signed-off-by: Alex Tomlins <alex@tomlins.org.uk>