Media: Ensure a consistent structure for the array returned by `wp_get_attachment_image_src()`.

The array returned by this function previously included a fourth element only when an intermediate image size was used. This change ensures a consistent structure in the returned array regardless of the function's internal behaviour.

Props wido, joemcgill, johnbillion

Fixes #46113
Built from https://develop.svn.wordpress.org/trunk@47748


git-svn-id: https://core.svn.wordpress.org/trunk@47524 1a063a9b-81f0-0310-95a4-ce76da25c4cd