[php-src] PR #23706: standard: Improve `sprintf()` error message when providing an empty arg specifier

Pull Request: https://github.com/php/php-src/pull/23706 Author: TimWolla I misremembered the correct syntax for specifying the argnum in an sprintf() format string and placed the number after the $ instead of before. This lead to sprintf() seeing an empty number and reporting: > Argument number specifier must be greater than zero and less than 2147483647 which was misleading. Had the error message shown that the argnum is empty, it would have been much clearer that it expect the number before the $.