[IMP] web: small optimization in calendar view

In some cases, the calendar view sends a trivial domain, such as
["partner_id", "not in", []]

This is clearly not really useful, and it could hurt performances in
some cases (for example, if the partner_id field is a related on a large
table, and non stored).

This is clearly not a complete fix, but it is easy, safe and does not
hurt.