Prevent fatal error when get_the_tag_list return wp_error object

[nirav7707]

Trac ticket

• https://core.trac.wordpress.org/ticket/60800

Implemented validation to ensure that the variable holding the return value of the function get_the_tag_list is not a WP_Error object in twenty twenty one theme.

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[github-actions]

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[SergeyBiryukov]

Thanks for the PR! Merged in r57991.