[FIX] stock: expected singleton issue in quants when lot/sn number

If applied, this commit will solve the issue of the singleton when there are 2
quants with the same location and same products but one quant with a lot and
another quant without a lot number.

Steps to produce:
- Create one quant with location-1 and without lot number.
- Create another quant with location-1 but with the lot number.
- Create 3rd quant with location-1 with same lot number as step2.
The error will raise in 3rd step as it is not accepting the 2 quants where one
is with lot number and another is without lot number.

see - https://tinyurl.com/2hqrgmwm



sentry - 4024572562

closes odoo/odoo#116318

Signed-off-by: Tiffany Chang <tic@odoo.com>