[bugfix] Don't reset Bot._site to None if we have already a site object
[bugfix] Don't reset Bot._site to None if we have already a site object

Bug: T125046
Change-Id: Id900d93b6235ea4ec60a0db97d7b7da2828fcaad

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core