fix php deprecated warning when passing null as subject in preg_match…

…() (serbanghita#879)

fix serbanghita#876

Mobile_Detect.php

@@ -1450,13 +1450,17 @@ public function is($key, $userAgent = null, $httpHeaders = null)
  * the User-Agent string.
  *
  * @param $regex
- * @param  string $userAgent
+ * @param string $userAgent
  * @return bool
  *
  * @todo: search in the HTTP headers too.
  */
  public function match($regex, $userAgent = null)
  {
+ if (!\is_string($userAgent) && !\is_string($this->userAgent)) {
+ return false;
+ }
+
  $match = (bool) preg_match(sprintf('#%s#is', $regex), (false === empty($userAgent) ? $userAgent : $this->userAgent), $matches);
  // If positive match is found, store the results for debug.
  if ($match) {