[FIX] stock: assign move

- Create a BOM for Product A, using Component B
  1 Unit of a is built with 1 Unit of B
- Update on hand quantity for B to 3.0
- Create a MO for 5 Units of A
- Update the quantity to produce to 3.0

The button 'Check Availability' never disappears.

The core of the problem is that the move remains "Partially Available".
Indeed, if there is no product available (`available_quantity <= 0`),
the stock move is skipped and its availability status is not updated.

We slightly change the logic by first checking if we need any quantity.
If not, the move can be considered as assigned.

opw-1850499