Add a test for iter::empty::<*mut i32> being Sync and Send

Author: KamilaBorowska

src/test/run-pass/threads-sendsync/sync-send-iterators-in-libcore.rs

@@ -91,6 +91,7 @@ fn main() {
     is_sync_send!((1..));
     is_sync_send!(repeat(1));
     is_sync_send!(empty::<usize>());
+    is_sync_send!(empty::<*mut i32>());
     is_sync_send!(once(1));
 
     // for option.rs