utils: Improve the return value of timestampToDate().

When constructing the empty string, it is better to use `std::string()` (or
just `{}` when returning a value) instead of `""` as the latter calls a
construct that needs to traverse the empty string literal, which is needless
pessimization.

src/utils/time.cpp

@@ -87,7 +87,7 @@ std::string getCurrentYear() {
 */
 std::string timestampToDate(std::tm *tm) {
 	if (tm == nullptr) {
-		return "";
+		return {};
 	}
 
 	std::stringstream ss;