[FIX] mrp: state on workorder with 0 batch size

Usecase:
- Go on a BoM with a routing. Go on the first operation and set
the next operation(batch field) as 'Once a minimum number of products is
processed'.
- Set the batch size (minimum operation) to 0.
- Create a MO and plan it.
- The second work order has state 'pending' instead of 'ready'

It happens because the system automatically set the first wo as
'ready' and the following as 'pending' and don't check the batch
and batch_size.

closes odoo/odoo#32628

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>