check also for null duration

src/Entities/Audio.php

@@ -34,7 +34,7 @@ class Audio extends Entity
         }
 
         $this->duration = isset($data['duration']) ? $data['duration'] : null;
-        if ($this->duration === '') {
+        if ($this->duration === '' || $this->duration === null) {
             throw new TelegramException('duration is empty!');
         }
 

src/Entities/Video.php

@@ -45,7 +45,7 @@ class Video extends Entity
             throw new TelegramException('height is empty!');
         }
         $this->duration = isset($data['duration']) ? $data['duration'] : null;
-        if ($this->duration === '') {
+        if ($this->duration === '' || $this->duration === null) {
             throw new TelegramException('duration is empty!');
         }
         $this->thumb = isset($data['thumb']) ? $data['thumb'] : null;

src/Entities/Voice.php

@@ -32,7 +32,7 @@ class Voice extends Entity
         }
 
         $this->duration = isset($data['duration']) ? $data['duration'] : null;
-        if ($this->duration === '') {
+        if ($this->duration === '' || $this->duration === null) {
             throw new TelegramException('duration is empty!');
         }