Bug 17510: enable use of subfield 0 in MMT actions

Bug 17510: enable use of subfield 0 in MMT actions

When adding MARC modification template (MMT) actions, if a subfield
value of 0 (zero) is entered it will not show up in the action's
description upon saving. Also, if you try to modify an authority or
biblio record using actions that refer to subfield 0 the procedure
will fail.

This patch fixes that.

Test plan:
0) Create a MARC modification template and add some actions to it,
   ideally testing all action types (Delete/Add new/Update existing
   or add new/Move/Copy/Copy and replace); make sure you input a 0
   (integer zero) in the text box for the subfield value(s).
1) As you save each action, observe that subfield 0 ($0) is missing
   from the action description.
2) Home > Tools > Batch record modification: try to modify a biblio-
   graphic record using the modification template you just created.
   Observe that you get an error in the system logs if you click on
   'Show MARC' or 'Modify selected records'.
3) Apply the patch.
4) Repeat steps 0-2. The subfield value ($0) should be displayed in
   the action description and the Batch record modification should
   work without problems.

Signed-off-by: Lisette Scheer <lisettes@latahlibrary.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>