:art: Do not set nonnull flag on unions

diff --git a/ast b/ast
index 1e502b836600cac639114f775ef7227c43cf8db7..cc3a1b18354ea5f3d06bfec3eb747ed3cec4a141 160000 (submodule)
--- a/ast
+++ b/ast
@@ -1 +1 @@
-Subproject commit 1e502b836600cac639114f775ef7227c43cf8db7
+Subproject commit cc3a1b18354ea5f3d06bfec3eb747ed3cec4a141