db constraint with unique remoteId if set, handle sync with tests

src/Bundle/ChillCalendarBundle/Entity/Invite.php

@@ -26,9 +26,10 @@ use Symfony\Component\Serializer\Annotation as Serializer;
  * The event/calendar in the user may have a different id than the mainUser. We add then fields to store the
  * remote id of this event in the remote calendar.
  *
- * @ORM\Table(name="chill_calendar.invite", indexes={
- *     @ORM\Index(name="idx_calendar_invite_remote", columns={"remoteId"})
- * })
+ * @ORM\Table(
+ *     name="chill_calendar.invite",
+ *     uniqueConstraints={@ORM\UniqueConstraint(name="idx_calendar_invite_remote", columns={"remoteId"}, options={"where": "remoteId <> ''"})}
+ * )
  * @ORM\Entity
  */
 class Invite implements TrackUpdateInterface, TrackCreationInterface