Replace `TokenStream::empty` with `::new`

`TokenStream::empty` is deprecated since `proc-macro2` 0.4.4.
diff --git a/codegen/src/main.rs b/codegen/src/main.rs
index 5c2f683..f00f310 100644
--- a/codegen/src/main.rs
+++ b/codegen/src/main.rs
@@ -934,7 +934,7 @@
                         semi_token: None,
                     }),
                 },
-                features: TokenStream::empty(),
+                features: TokenStream::new(),
                 eos_full: false,
             },
         );