Bug 28423: JavaScript error on MARC modifications page

Bug 28423: JavaScript error on MARC modifications page

This patch makes a minor change to the MARC modifications template so
that the "mmtas" variable isn't defined if there is no JSON to be
assigned as its value.

To test, apply the patch and go to Administration -> MARC modification
templates.

 - If necessary, add a template with at least one action.
 - Check the browser console, there should be no errors.
 - Click the "Edit" button corresponding to one of the template actions.
 - The details of the action should load correctly in the edit form and
   there should be no errors in the console.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>