Fix Dolibarr#35649 allow credit note with positive HT when TTC is zero or negative

The validation guard refused a credit note as soon as total_ht was strictly positive,
which blocked legitimate refunds of an invoice mixing a positive VAT line with a
0% VAT gift-card line (HT=+10, VAT=-10, TTC=0). Check total_ttc instead, mirroring
the symmetric guard for non credit-note invoices a few lines below, and keep the
existing error message.

Signed-off-by: Dolicraft <contact@dolicraft.com>

Author: Dolicraft

htdocs/compta/facture/card.php

@@ -411,7 +411,9 @@
 		// We check invoice sign
 		if ($object->type == Facture::TYPE_CREDIT_NOTE) {
 			// If a credit note, the sign must be negative
-			if ($object->total_ht > 0) {
+			// Note: total_ht can be positive when total_ttc is 0 or negative (mirror of the
+			// invoice case below), e.g. a refund of a 0 VAT gift card line cancelling a VAT line.
+			if ($object->total_ttc > 0) {
 				setEventMessages($langs->trans("ErrorInvoiceAvoirMustBeNegative"), null, 'errors');
 				$action = '';
 			}