Work around empty_docs clippy lint in test

    warning: empty doc comment
       --> tests/test_item.rs:174:17
        |
    174 |                 #[doc = $doc]
        |                 ^^^^^^^^^^^^^
    ...
    180 |     doc_expr!(stringify!());
        |     ----------------------- in this macro invocation
        |
        = help: consider removing or filling it
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#empty_docs
        = note: `-W clippy::empty-docs` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::empty_docs)]`
        = note: this warning originates in the macro `doc_expr` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/test_item.rs

@@ -177,7 +177,7 @@ mod test_doc_expr {
         };
     }
 
-    doc_expr!(stringify!());
+    doc_expr!(stringify!(...));
 
     #[test]
     fn test_doc_expr() {