LP#1414112: avoid excluding record attribute values that contain only blanks

LP#1414112: avoid excluding record attribute values that contain only blanks

Certain record attributes, chiefly the ones whose values comes
from MARC fixed fields and which have a coded value map associated
with them, can have a string consisting of one or more blanks as a
valid value.  Consequently, this patch ensures that
reingest_record_attributes() no longer excludes all attributes that
have blank values from the final attribute list.

This fixes a problem where MARC records with the target audience (008/22)
coded as a blank (unknown or not specified) could no longer be retrieved
using an "audience( )" search filter.

After applying this patch, a reingest of record attributes should be
performed, e.g., by doing

  select metabib.reingest_record_attributes(id)
  from biblio.record_entry
  where not deleted;

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>