Don't return symbols with empty names

Although LSP does not define what constitutes a valid symbol name, VS
Code's client implementation explicitly prohibits symbols with empty
names. Currently, RLS returns such symbols in many cases (for example,
the root module of a binary crate). These symbols not only fail to
parse in VS Code, but in fact cause it to throw an exception that
prevents the symbol list from being populated *at all*.

To fix this, filter out symbols with empty names before returning them
to the client.

src/actions/requests.rs

@@ -112,6 +112,7 @@ impl RequestAction for Symbols {
 
         Ok(symbols
             .into_iter()
+            .filter(|s| !s.name.is_empty())
             .filter(|s| {
                 let range = ls_util::rls_to_range(s.span.range);
                 range.start != range.end