Setting default minimumSeverity to zero from null
Setting default minimumSeverity to zero from null

Summary:
In hphp's version of idx, it uses debug_rlog() which is not
available in phabricator's code. It will be executed if null is passed
as the second value to idx.

Test Plan:
ran arc lint which still report error message, and php
./scripts/arcanist.php unit src/lint/linter/xhpast/__test__ doesn't
throw.

Reviewed By: epriestley
Reviewers: tuomaspelkonen, epriestley
CC: aran, jungejason, epriestley
Differential Revision: 666

Symplicity/arcanist,dguo-coursera/arcanist,aik099/arcanist,coursera/arcanist,wxstars/arcanist,uber/arcanist,memsql/arcanist,zhihu/arcanist,phacility/arcanist,eSpark/arcanist,hach-que/arcanist,eSpark/arcanist,memsql/arcanist,eSpark/arcanist,zhihu/arcanist,dguo-coursera/arcanist,alexmv/arcanist,KieranHayes/arcanist,phacility/arcanist,uber/arcanist,Symplicity/arcanist,wharris/arcanist,kevinseelbach/arcanist,chundongwang/arcanist,fivestars/arcanist,wikimedia/phabricator-arcanist,KieranHayes/arcanist,chundongwang/arcanist,fivestars/arcanist,Khan/arcanist,phacility/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,uber/arcanist,wharris/arcanist,sneakpeeq/arcanist,calblueprint/arcanist,calblueprint/arcanist,jra3/arcanist,aik099/arcanist,wharris/arcanist,Symplicity/arcanist,jra3/arcanist,wxstars/arcanist,Symplicity/arcanist,haskell-infra/arcanist,sneakpeeq/arcanist,codelabs-ch/arcanist,julkiewicz/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,memsql/arcanist,alexmv/arcanist,r4nt/arcanist,PageUpPeopleOrg/arcanist-patches,zhihu/arcanist,julkiewicz/arcanist,fivestars/arcanist,fivestars/arcanist,codelabs-ch/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,Khan/arcanist,folsom-labs/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,julkiewicz/arcanist,wxstars/arcanist,calblueprint/arcanist,PageUpPeopleOrg/arcanist-patches,sneakpeeq/arcanist,hach-que/arcanist,chundongwang/arcanist,coursera/arcanist,coursera/arcanist,phacility/arcanist,julkiewicz/arcanist,aik099/arcanist,KieranHayes/arcanist,phacility/arcanist,alexmv/arcanist,r4nt/arcanist,codelabs-ch/arcanist,r4nt/arcanist,aik099/arcanist,kevinseelbach/arcanist,codelabs-ch/arcanist,hach-que/arcanist,wikimedia/phabricator-arcanist,calblueprint/arcanist,uber/arcanist,KieranHayes/arcanist,Khan/arcanist,kevinseelbach/arcanist,coursera/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,folsom-labs/arcanist,jra3/arcanist,zhihu/arcanist,chundongwang/arcanist,hach-que/arcanist,alexmv/arcanist,kevinseelbach/arcanist,dguo-coursera/arcanist,sneakpeeq/arcanist,haskell-infra/arcanist,memsql/arcanist,folsom-labs/arcanist,r4nt/arcanist,wharris/arcanist,wxstars/arcanist,eSpark/arcanist,jra3/arcanist