LP#1116509: prevent null SVF attribute value from breaking record import

LP#1116509: prevent null SVF attribute value from breaking record import

Fix problem where a record could fail to be staged for import
with the following error:

ERROR: query string argument of EXECUTE is null
CONTEXT: PL/pgSQL function "extract_rec_attrs" line 72 at EXECUTE statement
SQL function "extract_rec_attrs" statement 1

This error would be triggered if at least one SVF attribute has a normalization
function defined that is applied to a null value during record import.

Patch is based on a suggestion by Bob Wicksall.

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