Correct pattern matching of nullable wrapped tag unions

The nullable ID always has zero tags. For everything else, we should
just match with the arity of the number of arguments, which doesn't
include the tag ID.

crates/compiler/test_mono/src/tests.rs

@@ -2314,3 +2314,29 @@ fn issue_4557() {
         "###
     )
 }
+
+#[mono_test]
+fn nullable_wrapped_with_non_nullable_singleton_tags() {
+    indoc!(
+        r###"
+        app "test" provides [main] to "./platform"
+
+        F : [
+            A F,
+            B,
+            C,
+        ]
+
+        g : F -> Str
+        g = \f -> when f is
+                A _ -> "A"
+                B -> "B"
+                C -> "C"
+
+        main =
+            g (A (B))
+            |> Str.concat (g B)
+            |> Str.concat (g C)
+        "###
+    )
+}