fix: skip warning for free items

[barredterra]

Our use case: we have a free item on the Delivery Note and also want that same free item on the mapped Sales Invoice.

For 0-amount line items we used to get the following warning: "System will not check over billing since amount for Item {item_code} in Delivery Note is zero."

Some users just get confused by this warning and stop working on the document, while there's nothing wrong with it.

Hence, I propose to silently skip overbilling checks for free items (0-amount on ref-doc and 0-amount on current doc).

Internal ref: LAN-868

[miteshpc]

This can also happen when a newly added item without an item price is added by the sales invoice user. Shouldn't there be an override at the line item level to avoid stock reduction by user error at 0 revenue, resulting into unexpected profits showing up at the cost of COGS.

Some kind of control is needed instead of letting 0 rate items gone unnoticed.