Before this commit, doing expression.OR() with only FALSE_LEAF would yield [] which is equivalent to TRUE_LEAF and is therefore not correct. The same happened (to a lesser extent) with expression.AND() within an expression.OR(), since the former would return a [] which would be ignored by expression.OR(). See tests for a clearer view of the use cases. Fixes #30113, #26540 closes odoo/odoo#31202