Bug 36494: Flatpickr error on checkout page if the patron is blocked from checking out

Bug 36494: Flatpickr error on checkout page if the patron is blocked from checking out

This patch add some error-handling to the recent change to
circulation.tt where we handle dates and on-site checkouts. This
prevents an error in the console when the patron is blocked and the
"specify due date" field is hidden.

To test, apply the patch and make sure OnSiteCheckouts is enabled.

- Go to Circulation and check out to a patron.
- Under the restrictions tab, add a manual restriction.
- When the page reloads and the checkout form is no longer visible,
  confirm that there is no error in the browser console.

Confirm that the Bug 18885 test plan still works.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>