Fix clang warnings on some utils function. It’s also faster

src/utils/get_first_non_empty.hpp

@@ -6,13 +6,13 @@ bool is_empty(const std::string& val);
 bool is_empty(const int& val);
 
 template <typename T>
-T get_first_non_empty(T&& last)
+T& get_first_non_empty(T&& last)
 {
   return last;
 }
 
 template <typename T, typename... Args>
-T get_first_non_empty(T&& first, Args&&... args)
+T& get_first_non_empty(T&& first, Args&&... args)
 {
   if (!is_empty(first))
     return first;