User can remove last unit of quantity from POL

Overview: With new workflow a user can navigate to receiving and delete the only Piece related to the order. This changes the POL quantity and cost to 0

Note mostly this doesn't seem to cause issues but I found at least the invoice won't pay. If there are other issues we may need to address those as well

Steps to Reproduce:

1. Log into folio-testing
2. Create order
3. Add POL with quantity of 1
4. AND manually add pieces for receiving = false
5. Open order
6. Navigate to receiving
7. Delete only piece
8. View POL
9. Create invoice for POL
10. Approve invoice
11. Pay invoice

Expected Results: When quantity equals 0 for a POL the estimated cost should = 0. Order can still be unopened or closed without error. encumbrance can be rolled over if needed. Invoices can be approved and paid with quantity 0 and cost 0.

Actual Results: Invoice can be approved but not paid.

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