Issue #3277342: Fix substr issue for php8.

Closes #3277342

In PHP 7 substr returns the false but in PHP 8 it returns the '' empty string. It produces an issue when site can't be loaded if JShrink is used.

Current solution keeps backward compatibility.

See: https://www.php.net/manual/en/function.substr.php

advagg_js_compress/jshrink.inc

@@ -322,7 +322,7 @@ class Minifier
             $char = substr($this->input, $this->index, 1);
 
             // If the next character doesn't exist return false.
-            if (isset($char) && $char === false) {
+            if ((string) $char === '') {
                 return false;
             }