Bug 37836: Prevent submitting empty barcodes in Self check-in

Bug 37836: Prevent submitting empty barcodes in Self check-in

This patch disbles the "Submit" button when the barcode field is empty.

To test this patch:

Add the patch to your koha clone
Enable the "SelfCheckInModule".
Open the page and the "Submit" button should be disabled when the barcode field is empty.

Signed-off-by: Sam Sowanick <sam.sowanick@corvallisoregon.gov>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>