bug 3520: fix crash when adding or editing items

bug 3520: fix crash when adding or editing items

Fix the following crash when adding or editing an
item record in the staff interface:

Can't call method "append_fields" on an undefined value at
/usr/local/share/perl/5.10.0/MARC/File/SAX.pm line 92.
 at /usr/local/share/perl/5.10.0/MARC/File/SAX.pm line 92

This crash appears only if a version of MARC::File::XML
greater than 0.88 is installed, and was triggered by
C4::Biblio::TransformHtmlToXml() failing to create a valid
MARCXML blob, which must include a <record> element.

This patch also fixes the indicator values generated by
TransformHtmlToXml(), setting them to " " instead of
"" when no indicator value is supplied.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>