Remove auto trait implementation section when empty

src/test/rustdoc/empty-section.rs

@@ -0,0 +1,20 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_name = "foo"]
+
+#![feature(optin_builtin_traits)]
+
+pub struct Foo;
+
+// @has foo/struct.Foo.html
+// @!has - '//*[@class="synthetic-implementations"]' 'Auto Trait Implementations'
+impl !Send for Foo {}
+impl !Sync for Foo {}