m/update: fix "not a reference at /../Utils.pm"

As Petter Reinholdtsen noticed :

    The issue is that $data in the call to Storable::nstore() is
    undefined.

    This get rid of the warning in the cron email triggered by trying to store
    nothing in /var/lib/munin/htmlconf.storable.tmp.$pid.  But the cron email
    is still sent out, this time with another and more understandable message:

      [FATAL] There is nothing to do here, since there are no nodes with
      any plugins. Please refer to http://munin-monitoring.org/wiki/FAQ_no_graphs

I commited a slighly edited patch, but the effect is exactly the same as the
one that Petter wrote.

Closes: D:696981
Merge branch 'bts/696981-fix-not-a-reference-utils-pm' into stable-2.0