Check the value passed to `get_post_type_object()`. If it's an array, use the first item. `get_query_var( 'post_type' )` can be an array if the query has been altered via filters/actions. There are several places in core that pass the query var. Adds unit tests.

In `template-loader.php`, move `is_post_type_archive()` and `is_tax()` directly below `is_home()`.

See #18614, [25291].


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