Contacts: Don't try to render invalid properties.

apps/contacts/lib/vcard.php

@@ -447,6 +447,7 @@ class OC_Contacts_VCard{
 		$details = array();
 		foreach($object->children as $property){
 			$temp = self::structureProperty($property);
+			if(!is_null($temp)) {
 				if(array_key_exists($property->name,$details)){
 					$details[$property->name][] = $temp;
 				}
@@ -454,6 +455,7 @@ class OC_Contacts_VCard{
 					$details[$property->name] = array($temp);
 				}
 			}
+		}
 		return $details;
 	}