Bug 38842: Add a op=cud-null to the form

This will make xt/find-missing-op-in-forms.t pass
But also enforce the use of a op=cud- parameter.

It is at the end of the form element, so if another is passed in the
modal body the one passed will get retrieved properly by CGI->param

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>

koha-tmpl/intranet-tmpl/prog/en/includes/html_helpers.inc

@@ -709,6 +709,7 @@
                             <button type="button" data-bs-dismiss="modal" class="btn btn-default" id="[% modal_id | html %]-ok">[% t('OK') | html %]</button>
                         [% END %]
                     </div>
+                    <input type="hidden" name="op" value="cud-null" /> [%# Must be passed in the content of the modal %]
                 </form>
             </div>
         </div>