Negative nested search on Many2One should return record with NULL value

When doing a negative search on a nested target, it is expected that record with the a target to be included in the result. This is important for when we have a searcher which return multiple clauses to different targets, we want to match record without one of the target. For example if we have domain [('name', '!=', 'foo')] which result in a domain like [('party1.name', '!=', 'foo'), ('party2.name', '!=', 'foo')] we want to find the record that have no party1 and party2.name != 'foo' (and vis-versa).