fix function fails if there are no sizes in db.

Extensions/WordpressTwigExtension.php

@@ -202,6 +202,8 @@ public function getThumbnail(Post $post, $size = 'thumbnail', $keepRatio = false
 
         $basename = $this->basename($thumbnail->getGuid());
         $nearestSize = $this->getNearestSize($thumbnail, $size, $keepRatio);
+
+        if ($nearestSize === null) return null;
 
         return str_replace($basename, $nearestSize['file'], $thumbnail->getGuid());
     }
@@ -225,6 +227,8 @@ private function getNearestSize($attachment, $target = array(300, 200), $keepRat
 
         list($x1, $y1) = $target;
 
+        if (!$allSizes) return null;
+
         do {
             foreach ($allSizes as $key => $size) {
                 $x2 = $size['width'];
@@ -260,7 +264,10 @@ private function getAllSizes($attachment)
             'file'   => $this->basename($attachment->getGuid()),
         );
 
-        $allSizes = $metadata['sizes'];
+        if (array_key_exists('sizes', $metadata)){
+            $allSizes = $metadata['sizes'];
+        }
+
         $allSizes['original'] = $originalSize;
 
         return $allSizes;