fix: deprecated php 8.4 nullable default value

Author: DrIsmaileAbdallah

src/OpenTok/OpenTok.php

@@ -1261,7 +1261,7 @@ public function dial($sessionId, $token, $sipUri, $options = [])
      *
      * @return void
      */
-    public function playDTMF(string $sessionId, string $digits, string $connectionId = null): void
+    public function playDTMF(string $sessionId, string $digits, ?string $connectionId = null): void
     {
         Validators::validateSessionIdBelongsToKey($sessionId, $this->apiKey);
         Validators::validateDTMFDigits($digits);

src/OpenTok/Util/Client.php

@@ -753,7 +753,7 @@ public function dial($sessionId, $token, $sipUri, $options)
         return $sipJson;
     }
 
-    public function playDTMF(string $sessionId, string $digits, string $connectionId = null): void
+    public function playDTMF(string $sessionId, string $digits, ?string $connectionId = null): void
     {
         $route = sprintf('/v2/project/%s/session/%s/play-dtmf', $this->apiKey, $sessionId);
         if ($connectionId) {