feat: add new tests

tests/programs/pos_null.fzs

@@ -0,0 +1,23 @@
+class Item
+{
+  int value;
+}
+
+fn maybe_item(bool create) Item
+{
+  if (create)
+    return new Item;
+
+  return null;
+}
+
+Item empty = maybe_item(false);
+Item present = maybe_item(true);
+if (empty == null && present != null)
+  printline("null handling passed");
+else
+{
+  printline("null handling failed");
+  exit(1);
+}
+