LP#1303940: don't attempt to store NULL values in keyword, browse, or facet indexes

LP#1303940: don't attempt to store NULL values in keyword, browse, or facet indexes

This patch implements a suggestion by Dan Wells and builds on Mike
Rylander's patch to skip inserting into field_entry and facet
indexes values that have been normalized to NULL.  As a side effect,
the following warning will no longer be logged when attempting
to store such values in the facet index:

WARNING:  Use of uninitialized value in subroutine entry at /usr/lib/perl/5.18/Unicode/Normalize.pm line 80.
CONTEXT:  PL/Perl function "force_unicode_normal_form"

Signed-off-by: Galen Charlton <gmc@esilibrary.com>