fix: check string offset exists before using it

Author: aprat84

src/Git/Commit/Footer.php

@@ -51,7 +51,7 @@ public function __construct(string $token, string $value)
         $tokenLower = strtolower($this->token);
         $values = preg_split('/[,\s]/', $this->value);
         foreach ($values as $val) {
-            if ($val[0] === '#') {
+            if (isset($val[0]) && $val[0] === '#') {
                 $ref = ltrim($val, '#');
                 $obj = new Reference($ref);
                 if (in_array($tokenLower, self::TOKEN_CLOSE_ISSUE)) {