Don't calculate the blurhash on images without a size

src/Object/Image.php

@@ -743,6 +743,10 @@ class Image
 			$height = $image->getHeight();
 		}
 
+		if (empty($width) || empty($height)) {
+			return '';
+		}
+
 		$pixels = [];
 		for ($y = 0; $y < $height; ++$y) {
 			$row = [];