Always use ElementAtOrDefault in ElementAtOrDefaultTests (#33578)
Always use ElementAtOrDefault in ElementAtOrDefaultTests (#33578)

It looks like ElementAtOrDefaultTests were copied from ElementAtTests, and not all the usages of ElementAt were changed to ElementAtOrDefault. This commit fixes that.
ericstj/corefx,wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,BrennanConroy/corefx,ViktorHofer/corefx,ericstj/corefx,ViktorHofer/corefx,ptoonen/corefx,ptoonen/corefx,wtgodbe/corefx,ViktorHofer/corefx,shimingsg/corefx,ViktorHofer/corefx,ViktorHofer/corefx,wtgodbe/corefx,BrennanConroy/corefx,wtgodbe/corefx,ptoonen/corefx,ptoonen/corefx,shimingsg/corefx,ericstj/corefx,wtgodbe/corefx,wtgodbe/corefx,wtgodbe/corefx,ericstj/corefx,shimingsg/corefx,ptoonen/corefx,ptoonen/corefx,shimingsg/corefx,shimingsg/corefx,BrennanConroy/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,ericstj/corefx,ptoonen/corefx