Comments: Check if reply heading text node exists before accessing its...

Comments: Check if reply heading text node exists before accessing its property in `comment-reply.js`.

Follow-up to [47506], [48876].

Props johannadevos, mailnew2ster.
Merges [48904] to the 5.5 branch.
Fixes #38009.
Built from https://develop.svn.wordpress.org/branches/5.5@48917


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48679 1a063a9b-81f0-0310-95a4-ce76da25c4cd