runtime: allow ranging over a nil map

This appears to be allowed by the specification, at least it is allowed
by the main Go implementation: https://play.golang.org/p/S8jxAMytKDB

Allow it in TinyGo too, for consistency.

Found because it is triggered with `tinygo test flags`. This doesn't
make the flags package pass all tests, but is a step closer.

testdata/map.go

@@ -49,6 +49,9 @@ func main() {
 	println(testmapIntInt[2])
 	testmapIntInt[2] = 42
 	println(testmapIntInt[2])
+	for k := range nilmap {
+		println(k) // unreachable
+	}
 
 	arrKey := ArrayKey([4]byte{4, 3, 2, 1})
 	println(testMapArrayKey[arrKey])