style: Latest clippy suggestions (#2048)

Have fixed a few new suggestions from the latest version of clippy.

src/test/mod.rs

@@ -89,7 +89,7 @@ impl<'a> ModuleRenderer<'a> {
         // convention was that there would be no module but None. This is nowadays not anymore
         // the case (to get durations for all modules). So here we make it so, that an empty
         // module returns None in the tests...
-        ret.filter(|s| s != "")
+        ret.filter(|s| !s.is_empty())
     }
 }