[FIX] stock: Support for optional components in kit

When a BoM is using a component with a quantity required set to 0,
tracebacks were occuring when computing the available quantities
for the kit.

Fixes #38807

closes odoo/odoo#39267

X-original-commit: 7676269b
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>