[Runtime] Make debug builds accept NULL in swift_getObjCClassMetadata.
[Runtime] Make debug builds accept NULL in swift_getObjCClassMetadata.

The previous fix here switched dyn_cast for dyn_cast_or_null, but this left us with an assertion failure in cast<>. Instead, explicitly check for NULL at the top of the function.

JGiola/swift,rudkx/swift,gregomni/swift,parkera/swift,parkera/swift,xwu/swift,tkremenek/swift,gregomni/swift,apple/swift,JGiola/swift,ahoppen/swift,JGiola/swift,roambotics/swift,parkera/swift,tkremenek/swift,roambotics/swift,hooman/swift,xwu/swift,ahoppen/swift,glessard/swift,JGiola/swift,rudkx/swift,atrick/swift,gregomni/swift,apple/swift,glessard/swift,parkera/swift,hooman/swift,hooman/swift,atrick/swift,glessard/swift,apple/swift,hooman/swift,apple/swift,glessard/swift,hooman/swift,apple/swift,ahoppen/swift,tkremenek/swift,JGiola/swift,roambotics/swift,gregomni/swift,xwu/swift,atrick/swift,parkera/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,roambotics/swift,rudkx/swift,ahoppen/swift,parkera/swift,glessard/swift,gregomni/swift,tkremenek/swift,roambotics/swift,benlangmuir/swift,xwu/swift,JGiola/swift,atrick/swift,tkremenek/swift,xwu/swift,roambotics/swift,hooman/swift,xwu/swift,atrick/swift,tkremenek/swift,gregomni/swift,parkera/swift,benlangmuir/swift,parkera/swift,benlangmuir/swift,glessard/swift,rudkx/swift,rudkx/swift,hooman/swift,benlangmuir/swift,rudkx/swift,apple/swift,xwu/swift,ahoppen/swift,tkremenek/swift,benlangmuir/swift