if failureReason is nil, send @"" instead of [NSNull null]
if failureReason is nil, send @"" instead of [NSNull null]

Summary:
This fixes a crash in xcodetool --
https://phabricator.fb.com/P2092156

There were places in the TextReporter and PhabricatorReporter code that
assumed event[@"failureReason"] was always a string.  Sometimes it was
[NSNull null] instead.

Test Plan:
hard-coded event[@"failureReason"] to be @"" in xcocebuild-lib and
verified that the reporters were cool with an empty string being there -
they were.

Richardlihui/xctool,kernandreas/xctool,agreco/xctool,soundcloud/xctool,facebook/xctool,FiftyThree/xctool,facebook/xctool,LegNeato/xctool,kcoons/xctool,trello/xctool,Richardlihui/xctool,Monits/xctool,blighli/xctool,facebook/xctool,LegNeato/xctool,bgertzfield/xctool,agreco/xctool,lpios/xctool,lqi/xctool,kcoons/xctool,LegNeato/xctool,bhamiltoncx/xctool,dvor/xctool,SuPair/xctool,soundcloud/xctool,modulexcite/xctool,lqi/xctool,Monits/xctool,githubdelegate/xctool,ekingo/xctool,FiftyThree/xctool,bgertzfield/xctool,Monits/xctool,bgertzfield/xctool,bgertzfield/xctool,jspahrsummers/xctool,adamjernst/xctool,duk42111/xctool,lqi/xctool,agreco/xctool,1yvT0s/xctool,ACChe/xctool,n054/xctool,dvor/xctool,bhamiltoncx/xctool,trello/xctool,leopardpan/xctool,n054/xctool,trello/xctool,theiren46/xctool,facebook/xctool,jspahrsummers/xctool,kamoljan/xctool,theiren46/xctool,AutoScout24/xctool,FiftyThree/xctool,Monits/xctool,blighli/xctool,ACChe/xctool,1yvT0s/xctool,bgertzfield/xctool,n054/xctool,duk42111/xctool,dvor/xctool,bhamiltoncx/xctool,modulexcite/xctool,ACChe/xctool,lpios/xctool,AutoScout24/xctool,adamjernst/xctool,yaoxiaoyong/xctool,AutoScout24/xctool,facebook/xctool,ekingo/xctool,FiftyThree/xctool,kernandreas/xctool,lpios/xctool,kcoons/xctool,leopardpan/xctool,soundcloud/xctool,soundcloud/xctool,bhamiltoncx/xctool,jspahrsummers/xctool,ACChe/xctool,yaoxiaoyong/xctool,modulexcite/xctool,SuPair/xctool,bhamiltoncx/xctool,soundcloud/xctool,lpios/xctool,githubdelegate/xctool,theiren46/xctool,githubdelegate/xctool,blighli/xctool,lpios/xctool,jspahrsummers/xctool,PSPDFKit-labs/xctool,n054/xctool,ekingo/xctool,ACChe/xctool,duk42111/xctool,bgertzfield/xctool,n054/xctool,1yvT0s/xctool,PSPDFKit-labs/xctool,Richardlihui/xctool,bhamiltoncx/xctool,facebook/xctool,PSPDFKit-labs/xctool,SuPair/xctool,soundcloud/xctool,kernandreas/xctool,kamoljan/xctool,theiren46/xctool,dvor/xctool,ekingo/xctool,PSPDFKit-labs/xctool,0xced/xctool,agreco/xctool,Richardlihui/xctool,githubdelegate/xctool,leopardpan/xctool,duk42111/xctool,adamjernst/xctool,LegNeato/xctool,n054/xctool,0xced/xctool,AutoScout24/xctool,SuPair/xctool,adamjernst/xctool,leopardpan/xctool,kamoljan/xctool,ACChe/xctool,kernandreas/xctool,lpios/xctool,yaoxiaoyong/xctool,blighli/xctool,yaoxiaoyong/xctool,modulexcite/xctool,1yvT0s/xctool,kamoljan/xctool,trello/xctool,PSPDFKit-labs/xctool