[FIX] base: depends if no depends is ()

With saas-11.2's 3fbd86bc some trigger towards other fields are improved
but this caused an issue when a inherited field towards a custom
computed field had no depends.

When we created a computed field x_custo on res.partner, we have a
inherited x_custo on res.users automatically created.

Both these fields have a depends computed to `('',)`, this caused an
error for the one on res.partner: but this was ignored with dd048ae7.

Since saas-11.2's 3fbd86bc this also caused an error on the inherited
field but since an inherited field is not "manual" the error was not
ignored. Thus it was not possible to create custom field on a model
inheriting of another model if the fields had no depends.

opw-1854786
closes #25146

Co-authored-by: Raphael Collet <rco@odoo.com>