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

`TokenStream::empty` is deprecated since `proc-macro2` 0.4.4.
diff --git a/tests/test_generics.rs b/tests/test_generics.rs
index 9783b13..a8c4881 100644
--- a/tests/test_generics.rs
+++ b/tests/test_generics.rs
@@ -52,7 +52,7 @@
                     pound_token: Default::default(),
                     style: AttrStyle::Outer,
                     path: ident("may_dangle").into(),
-                    tts: TokenStream::empty(),
+                    tts: TokenStream::new(),
                     is_sugared_doc: false,
                 }],
                 ident: ident("T"),