Added return types to remove deprecated warning.

Author: stefanak-michal

src/packstream/Bytes.php

@@ -31,15 +31,15 @@ public function offsetGet($offset): ?string
         return $this->bytes[$offset] ?? null;
     }
 
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         if ($offset === null)
             $this->bytes[] = $value;
         else
             $this->bytes[$offset] = $value;
     }
 
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->bytes[$offset]);
     }