[FIX] base: don't break search in m2m with an empty right hand side

Operators child_of and parent_of fail if the right hand side is empty or yields empty results for many2many fields. We should avoid the exception generated by writing a query of the form ... id in ().

Closes https://github.com/odoo/odoo/pull/30121



closes odoo/odoo#38380

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>
Co-authored-by: mreficent <miquel.raich@eficent.com>