[FIX] mass_mailing: correctly check mailing domain

Currently each change in mailing model resets the mailing domain. Idea behind
that behavior is that domain for a given model will probably not be valid for
another model because of fields being model specific.

However this breaks when creating mailing with default values for mailing
model. Indeed even valid domains are reset to a default void / opt-out-ed
value due to current onchange implementation.

This commit fixes that behavior by resetting only invalid domain i.e.
that crashes when performing a search on target model.

LINKS

Task 1978729
PR #35061