fix: convert the nullable to be explicit

fix the follow deprecated messages at PHP 8.4:

PHP Deprecated:  Jeidison\JSignPDF\Sign\exec(): Implicitly marking parameter $output as nullable is deprecated, the explicit nullable type must be used instead in /home/runner/work/jsignpdf-php/jsignpdf-php/tests/JSignPDFTest.php on line 5
PHP Deprecated:  Jeidison\JSignPDF\Sign\exec(): Implicitly marking parameter $return_var as nullable is deprecated, the explicit nullable type must be used instead in /home/runner/work/jsignpdf-php/jsignpdf-php/tests/JSignPDFTest.php on line 5

Signed-off-by: Vitor Mattos <[email protected]>

Author: vitormattos

tests/JSignPDFTest.php

@@ -2,7 +2,7 @@
 
 namespace Jeidison\JSignPDF\Sign;
 
-function exec(string $command, array &$output = null, int &$return_var = null) {
+function exec(string $command, array &$output = null, ?int &$return_var = null) {
     global $mockExec;
     if ($mockExec) {
         $output = $mockExec;