Extend test_syntax to check that canonicalization doesn't panic on the input

Importantly, this would have caught the issue discovered in #7279, prior to that landing on main.

crates/compiler/test_syntax/fuzz/fuzz_targets/fuzz_expr.rs

@@ -11,7 +11,7 @@ fuzz_target!(|data: &[u8]| {
         let ast = input.parse_in(&arena);
         if let Ok(ast) = ast {
             if !ast.is_malformed() {
-                input.check_invariants(|_| (), true);
+                input.check_invariants(|_| (), true, None);
             }
         }
     }