Basic: guard against `nullptr` for `env`
Basic: guard against `nullptr` for `env`

`env` is a default'ed parameter.  In some cases it may be passed a `nullptr`,
which is not a valid parameter to `llvm::toStringRefArray`.  Guard against this
case.  This allows the swift compiler to work again on Windows!

harlanhaskins/swift,gribozavr/swift,gribozavr/swift,danielmartin/swift,danielmartin/swift,gregomni/swift,danielmartin/swift,jmgc/swift,airspeedswift/swift,allevato/swift,hooman/swift,JGiola/swift,glessard/swift,roambotics/swift,roambotics/swift,rudkx/swift,gribozavr/swift,lorentey/swift,austinzheng/swift,stephentyrone/swift,allevato/swift,xedin/swift,roambotics/swift,aschwaighofer/swift,atrick/swift,harlanhaskins/swift,parkera/swift,JGiola/swift,devincoughlin/swift,devincoughlin/swift,hooman/swift,jmgc/swift,aschwaighofer/swift,alblue/swift,practicalswift/swift,amraboelela/swift,xwu/swift,shajrawi/swift,aschwaighofer/swift,alblue/swift,devincoughlin/swift,ahoppen/swift,amraboelela/swift,xwu/swift,apple/swift,amraboelela/swift,apple/swift,brentdax/swift,alblue/swift,aschwaighofer/swift,alblue/swift,gribozavr/swift,jmgc/swift,ahoppen/swift,glessard/swift,atrick/swift,CodaFi/swift,gregomni/swift,austinzheng/swift,devincoughlin/swift,stephentyrone/swift,karwa/swift,benlangmuir/swift,brentdax/swift,nathawes/swift,hooman/swift,karwa/swift,jckarter/swift,amraboelela/swift,aschwaighofer/swift,xwu/swift,parkera/swift,shahmishal/swift,shahmishal/swift,JGiola/swift,lorentey/swift,atrick/swift,allevato/swift,allevato/swift,stephentyrone/swift,shahmishal/swift,JGiola/swift,ahoppen/swift,harlanhaskins/swift,devincoughlin/swift,benlangmuir/swift,karwa/swift,devincoughlin/swift,allevato/swift,benlangmuir/swift,airspeedswift/swift,atrick/swift,benlangmuir/swift,stephentyrone/swift,devincoughlin/swift,jckarter/swift,benlangmuir/swift,xwu/swift,karwa/swift,rudkx/swift,alblue/swift,jckarter/swift,harlanhaskins/swift,parkera/swift,apple/swift,shajrawi/swift,jmgc/swift,brentdax/swift,practicalswift/swift,stephentyrone/swift,hooman/swift,rudkx/swift,gregomni/swift,xedin/swift,stephentyrone/swift,amraboelela/swift,aschwaighofer/swift,harlanhaskins/swift,lorentey/swift,JGiola/swift,lorentey/swift,tkremenek/swift,glessard/swift,airspeedswift/swift,roambotics/swift,parkera/swift,rudkx/swift,parkera/swift,apple/swift,roambotics/swift,glessard/swift,xedin/swift,lorentey/swift,brentdax/swift,airspeedswift/swift,practicalswift/swift,parkera/swift,CodaFi/swift,ahoppen/swift,sschiau/swift,airspeedswift/swift,karwa/swift,harlanhaskins/swift,hooman/swift,tkremenek/swift,CodaFi/swift,parkera/swift,shajrawi/swift,sschiau/swift,tkremenek/swift,sschiau/swift,apple/swift,lorentey/swift,harlanhaskins/swift,austinzheng/swift,ahoppen/swift,shajrawi/swift,shahmishal/swift,sschiau/swift,gribozavr/swift,sschiau/swift,JGiola/swift,danielmartin/swift,aschwaighofer/swift,jmgc/swift,shajrawi/swift,hooman/swift,jckarter/swift,danielmartin/swift,alblue/swift,practicalswift/swift,jckarter/swift,apple/swift,nathawes/swift,karwa/swift,allevato/swift,glessard/swift,ahoppen/swift,nathawes/swift,shajrawi/swift,CodaFi/swift,xwu/swift,nathawes/swift,nathawes/swift,roambotics/swift,jmgc/swift,CodaFi/swift,xwu/swift,practicalswift/swift,karwa/swift,airspeedswift/swift,austinzheng/swift,practicalswift/swift,shahmishal/swift,brentdax/swift,xedin/swift,sschiau/swift,CodaFi/swift,xedin/swift,xwu/swift,gregomni/swift,sschiau/swift,xedin/swift,xedin/swift,allevato/swift,danielmartin/swift,austinzheng/swift,atrick/swift,stephentyrone/swift,atrick/swift,tkremenek/swift,austinzheng/swift,amraboelela/swift,shajrawi/swift,gribozavr/swift,gribozavr/swift,xedin/swift,hooman/swift,nathawes/swift,rudkx/swift,austinzheng/swift,tkremenek/swift,shajrawi/swift,practicalswift/swift,jckarter/swift,shahmishal/swift,tkremenek/swift,gribozavr/swift,devincoughlin/swift,shahmishal/swift,glessard/swift,alblue/swift,gregomni/swift,brentdax/swift,amraboelela/swift,rudkx/swift,CodaFi/swift,gregomni/swift,lorentey/swift,practicalswift/swift,karwa/swift,parkera/swift,airspeedswift/swift,lorentey/swift,sschiau/swift,jmgc/swift,brentdax/swift,jckarter/swift,shahmishal/swift,benlangmuir/swift,nathawes/swift,tkremenek/swift,danielmartin/swift