[FIX] stock : wrong quantity with transit location

- create a second warehouse, resupply from the first
- create a product, replenish W2 from W1
- Goto Inventory Forecast

--> Issue value are wrong.

An OUT is when WH source is Set and WH dest is not set.
An IN is when WH dest is Set and WH source is not set.

closes odoo/odoo#67598

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>