REST API: Include fields with null schema in `get_fields_for_response()`.

In [43736], we prevented rendering fields when not present in `?_fields=`. However, because `get_fields_for_response()` is dependent on `get_item_schema()`, any custom fields registered with a null schema would be incorrectly excluded from the response. Because the REST API permits a null schema for `register_rest_field()`, those fields should be included in the available fields for a response.

Props danielbachhuber.
Merges [43908] to trunk.
Fixes #45220.
Built from https://develop.svn.wordpress.org/trunk@44254


git-svn-id: https://core.svn.wordpress.org/trunk@44084 1a063a9b-81f0-0310-95a4-ce76da25c4cd