Dockingport gap linking fixes. Closes #551

- Dockingports force the hatch/door gaps to be connected to the hulls between the ports, because connecting them automatically may fail if the ports are positioned in a non-standard way.
- Gaps don't recheck hulls in OnMapLoaded if rechecks have been disabled.
- Item.UpdateHulls and Gap.UpdateHulls are not called when OnMapLoaded is called during ruin generation.

Barotrauma/BarotraumaShared/Source/Map/MapEntity.cs

@@ -391,8 +391,11 @@ namespace Barotrauma
                 mapEntityList[i].OnMapLoaded();
             }
             
-            Item.UpdateHulls();
-            Gap.UpdateHulls();
+            if (sub != null)
+            {
+                Item.UpdateHulls();
+                Gap.UpdateHulls();
+            }
 
             foreach (LinkedSubmarine linkedSub in linkedSubs)
             {