fix(web): fix compilation (#670)

Make tonic::body::empty_body public and use it instead of the old and non existant anymore BoxBody::empty()

tonic/src/body.rs

@@ -7,6 +7,7 @@ pub type BoxBody = http_body::combinators::BoxBody<bytes::Bytes, crate::Status>;
 
 // this also exists in `crate::codegen` but we need it here since `codegen` has
 // `#[cfg(feature = "codegen")]`.
-pub(crate) fn empty_body() -> BoxBody {
+/// Create an empty `BoxBody`
+pub fn empty_body() -> BoxBody {
     http_body::Empty::new().map_err(|err| match err {}).boxed()
 }