[FIX] hr_expense: expense lines > and < 0

- Create an expense sheet
- Line 1 is 500
- Line 2 is -200
- The total is 300 => a payment of 300 is made

When the payment is validated, the expense is not set as paid.

In v10, one expense line = one account move. In order to know if the
expense is paid we check that each account move is completely
reconciled. The account move of 500 is never completely reconciled by
the payment of 300, so the expense stays opened.

This simply cannot be supported in v10, since each expense line is
handled in a separate account move.

opw-740492