ci: fix phpstan error

Author: vincent4vx

src/Sprite/Sprite.php

@@ -244,6 +244,7 @@ private function doRotate(GdImage $img, int $angle): GdImage
         imagesavealpha($img, true);
         imagepng($img);
         $data = ob_get_clean();
+        assert($data !== false);
 
         $im = new \Imagick();
         $im->readImageBlob($data);