Protect against empty default_CD_modifiers in opensrf.xml

Protect against empty default_CD_modifiers in opensrf.xml

If the app settings for the open-ils.search service in opensrf.xml had
an empty (but not entirely missing) <default_CD_modifiers> element, then
the open-ils.storage log would contain entries like the following:

Use of uninitialized value $class in hash element at
/usr/local/share/perl5/OpenILS/Application/Storage/QueryParser.pm

Protect against that problem with more defensive code in QueryParser.pm.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Dan Scott <dan@coffeecode.net>