Bug 23868: Check if threshold is undefined in javascript block of opac-account.tt

Bug 23868: Check if threshold is undefined in javascript block of opac-account.tt

This patch fixes a bug introduced by checking payment plugins threshold

To test:
1. do not apply this patch
2. configure paypal options in intranet
3. create a manual invoice for a patron
4. in opac try to pay that invoice
CHECK => Make Payment button is disabled, even when invoices where checked.
5. apply this patch
6. repeat step 4
SUCCESS => Make Payment button is enabled when an invoice is checked.
7. Sign off

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>