bugfix error if memcached_servers is empty in koha-conf.xml

bugfix error if memcached_servers is empty in koha-conf.xml

XML::Simple returns by default an empty hash ref if it encounters an xml tag
with no attributes or content.  If memcached_servers is empty, as the installer
sets it when this feature is disabled, then we get the empty hash ref.

In checking if memcached_servers is set the empty hash ref evaluates as true.
I.E.

  $servers = C4::Context->config('memcached_servers');
  if ($servers) {

so we get a comiler error from memcached trying to resolve the nonexistant
server address.