Comments: Correct the check for reply element existence in `comment-reply.js`.

`document.getElementById()` returns `null` if no matching element was found, so the previous comparison didn't work as expected.

Follow-up to [47506].

Props mailnew2ster, sarahricker.
Merges [48876] to the 5.5 branch.
Fixes #38009.
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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48639 1a063a9b-81f0-0310-95a4-ce76da25c4cd