core: handle price 0 or None in Inventory API

The methods used in indexing did not make a difference between the case
where the price is None (ie. price is not set) or where the price is
set to 0 - in both cases this was saved as NULL in stock.total_price
column. This is incorrect, we need to keep the distinction between
these two cases also for inventory calculation.
We had some places where we select IFNULL(stock.total_price, 0) to work
around this, we don't plan to change the existing ones for now, but
while discussing on !1974
we concluded that a newly idenfified case of a problem consequence of
these NULL should be handled by fixing the indexation. To benefit from
the fix, impacted instances will have to reindex documents present in
the stock table with stock.total_price is null.