Issue #3273143: Allow values of zero in XFDF

Fixes issue introduced in 8da26cad.

src/Component/Utility/Xfdf.php

@@ -37,7 +37,7 @@ class Xfdf {
       $field_ele->setAttribute('name', $name);
 
       $value_ele = $field_ele->appendChild($doc->createElement('value'));
-      $value_ele->appendChild($doc->createTextNode($value ?: ''));
+      $value_ele->appendChild($doc->createTextNode((string) $value));
     }
 
     $ids_ele = $xfdf_ele->appendChild($doc->createElement('ids'));