LP#1254146 gracefully handle custom trees sans entries

Avoid references to custom tree entries when no tree entries for a
custom tree exist.  Prevents:

egweb: template error: undef error - Can't call method "org_unit" on an
undefined value at
/usr/local/share/perl/5.10.1/OpenILS/WWW/EGCatLoader/Util.pm line 185.\n

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>