Support for empty "Item" in invoice creation

When creating an invoice, if I leave the “Item” field empty, and save the invoice, this error is shown, “The Item field is required”. If I just put a “Space” character it gets around this issue, but I would like to know where it’s coming from.

I was looking at table ip_invoice_items , the item_name there is allowed to be Null so I’m at a loss of where this check is being done,

Appreciate any help.