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.
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