Spurious "empty trait list in `derive`" warnings with 1.14.0-nightly (ad19c32a5 2016-10-06)

With an old-style `Foo` derive plugin, `#[derive(Foo)]` triggers a warning about an empty trait list.

For example in Servo:

```
warning: empty trait list in `derive`
   --> /Users/nox/src/servo/components/script/script_runtime.rs:115:3
    |
115 | #[derive(JSTraceable, HeapSizeOf)]
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
