Add a deprecation cycle for `NullColumn` from `column_for_attribute`

This is public API, and `simple_form` depends on the `nil` return value.
We need to go through a deprecation cycle to return a null object. If
people want hash access, they can access the hash.
Merge pull request #15878 from sgrif/sg-null-column

Revert #15502, and add a deprecation warning instead.