Voucher import: Validate code not empty

src/pretix/base/modelimport_vouchers.py

@@ -44,6 +44,8 @@ class CodeColumn(ImportColumn):
         super().__init__(*args)
 
     def clean(self, value, previous_values):
+        if not value:
+            raise ValidationError(_('A voucher cannot be created without a code.'))
         if value:
             MinLengthValidator(5)(value)
         if value and (value in self._cached or Voucher.objects.filter(event=self.event, code=value).exists()):