LP#1435494: do not encourage <resultset_limit></resultset_limit>

LP#1435494: do not encourage <resultset_limit></resultset_limit>

An empty node in opensrf.xml gets parsed as an empty hashref,
not an empty scalar, so we'll use <resultset_limit>0</resultlet_limit>
in the suggested opensrf.xml config.  This fixes an issue
noticed by Ben Shum during testing where reports could fail with
the following message:

  DBD::Pg::st execute failed: ERROR: syntax error at or near "0"
  LINE 43: ) limited_to_HASH(0x2a974f8)_hits LIMIT HASH(0x2a974f8)
                             ^ at /openils/bin/clark-kent.pl line 243.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>