Fix broken issue update page due to custom field

In some corner cases, the customer hits invalid query which is trying to set
the custom field string to null. This is caused in the following case:

• Custom field is visible on update page.
• Custom field is not required.
• Custom field is of type list.
• Custom field doesn't have any value selected.

In this case, the new custom field value evaluated to default value provided by
bug_update.php page which is null. The fix is to change it to ''.

Fixes 0020196