fix formatting with empty compiled format string (#2042)

include/fmt/compile.h

@@ -643,7 +643,7 @@ FMT_INLINE std::basic_string<typename S::char_type> format(const S&,
 #ifdef __cpp_if_constexpr
   if constexpr (std::is_same<typename S::char_type, char>::value) {
     constexpr basic_string_view<typename S::char_type> str = S();
-    if (str.size() == 2 && str[0] == '{' && str[1] == '}')
+    if constexpr (str.size() == 2 && str[0] == '{' && str[1] == '}')
       return fmt::to_string(detail::first(args...));
   }
 #endif