Page is computed as "probably readerable" but parsing results in no content

Originally filed at https://bugzilla.mozilla.org/show_bug.cgi?id=1958633.

The page in question is: https://digital.tnconservationist.org/publication/?i=663361&article_id=3697028&view=articleBrowser

It appears that `Readerable.isProbablyReaderable(document)` returns `true`, but `Readability` parses the document and finds nothing to read.