missing fix for PHP 8.4 on StringEval::populate()

[ykawasaki683]

I found this error blow.

PHP Deprecated:  Smarty\Resource\StringEval::populate(): Implicitly marking parameter $_template as nullable is deprecated, the explicit nullable type must be used instead in /var/www/ec2/front/vendor/smarty/smarty/src/Resource/StringEval.php on line 34

This error disappeared by fixing this line like below.

smarty/src/Resource/StringEval.php

Line 34 in 8fd949a

public function populate(\Smarty\Template\Source $source, \Smarty\Template $_template = null)

public function populate(\Smarty\Template\Source $source, ?\Smarty\Template $_template = null)

On this fix, I referred this PR.
#1043