Comments: Rename `Walker_Comment::comment_text()` to `::filter_comment_text()` for clarity.

Ensure the comment object is not null before checking its status.

Follow-up to [47887].

See #49956.
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git-svn-id: https://core.svn.wordpress.org/trunk@47663 1a063a9b-81f0-0310-95a4-ce76da25c4cd