Fix page_tests testBasePageConstructor
Fix page_tests testBasePageConstructor

Empty string titles for BasePage raise InvalidTitle during Link.parse

Change-Id: I6d4fb9ff02d472f2e71933084de71a7bc355c154

wikimedia/pywikibot-core,icyflame/batman,trishnaguha/pywikibot-core,jayvdb/pywikibot-core,hasteur/g13bot_tools_new,PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,magul/pywikibot-core,Darkdadaah/pywikibot-core,smalyshev/pywikibot-core,h4ck3rm1k3/pywikibot-core,npdoty/pywikibot,xZise/pywikibot-core,jayvdb/pywikibot-core,TridevGuha/pywikibot-core,emijrp/pywikibot-core,VcamX/pywikibot-core,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,happy5214/pywikibot-core,wikimedia/pywikibot-core,magul/pywikibot-core,valhallasw/pywikibot-core,Darkdadaah/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,darthbhyrava/pywikibot-local