Code Modernization: Fix `rest_convert_error_to_response()` handling of non-array error data when obtaining `status`.

While the error data is normally an array, this is not guaranteed. This issue would have been detected by PHPStan rule level 9, since `WP_Error::get_all_error_data()` returns `mixed[]`.

Developed in https://github.com/WordPress/wordpress-develop/pull/11307

Follow-up to r61429.

Props nateallen, desrosj, jorbin, mukesh27, westonruter.
See #63430.
Fixes #64901.


git-svn-id: https://develop.svn.wordpress.org/trunk@62107 602fd350-edb4-49c9-b593-d223f7449a82