Query: Check that taxonomy query var is a string in `WP_Query::parse_tax_query()`.

This prevents a fatal error from `urldecode()` in `wp_basename()` if an array is passed instead.

Follow-up to [15732], [15824], [15825], [15923], [16155], [50565], [60927].

Props patricedefago, alexodiy, SergeyBiryukov.
Fixes #64870.

git-svn-id: https://develop.svn.wordpress.org/trunk@62047 602fd350-edb4-49c9-b593-d223f7449a82