[libSyntax] Don't set hot use memory region if source is empty
[libSyntax] Don't set hot use memory region if source is empty

If the source is empty, the start of the copied buffer is a nullptr which doesn't live inside the SyntaxArena's bump allocator. Thus the assert inside `setHotUseMemoryArea` fails.

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