When editing terms, properly display an admin `notice-warning` when `tag_ID` is empty.

Also, avoids two related PHP notices when empty or not set (and thus the global `$tag` is a WP_Error object).

Fixes: #34515.
Built from https://develop.svn.wordpress.org/trunk@35875


git-svn-id: https://core.svn.wordpress.org/trunk@35839 1a063a9b-81f0-0310-95a4-ce76da25c4cd