[bugfix] Relax assertion in WikidataBot.treat_page
[bugfix] Relax assertion in WikidataBot.treat_page

This assertion was wrong. page may be None,
it is even assigned to it above.

Change-Id: I05f4fd713625ded61bee7efe5e0c30d45b582b2d

wikimedia/pywikibot-core,wikimedia/pywikibot-core