User import: 'is_active' should have the default true

[Elblinator]

Current behaviour:
If a user is created via the import and the cell in the is_active row was empty then the backend saved the value "null"

Wanted behaviour:
the new default should be true (every user who is created via the import should have is_active:true, if not explicitly stated otherwise

For the needed migration:
If a user was imported like described, then this user cannot login and they get the snackbar Error

Error: The account is deactivated.

[Elblinator]

waiting for
OpenSlides/openslides-client#4011

[luisa-beerboom]

So... should the migration just set every user with is_active==None to True?

[rrenkert]

@luisa-beerboom Yes, every user that has is_active==None should be set to true.

[rrenkert]

Wanted behaviour: the new default should be true (every user who is created via the import should have is_active:true, if not explicitly stated otherwise

The default should always be used not only in the import.