[IMP] ORM: Coalesce NULL boolean values to false when generating ORDER BY

After commit f28be81, boolean columns may have more
NULL entries than before. In the (rare) cases where
a boolean column was used for an ORDER clause
(e.g. in the /shop page of website_sale), this
causes a change of the resulting ordering.

By coalescing NULL values to false in SQL,
we make the ordering consistent with what the
framework does for domain expressions with booleans,
and when reading boolean values, that is, NULL is
the same as False.