Previously if somehow a user created a blank markdown document using tag
tricks (eg `<p></p><p></p><p></p><p></p><p></p><p></p>`) and so on, we would
completely strip the document down to blank on post process due to onebox
hack.
Needs a followup cause I am still unclear about the reason for empty p stripping
and it can cause some unclear cases when we re-cook posts.