Always return a TravelAgent; Fixes BUKKIT-3541

Recent changes caused PlayerPortalEvent to suddenly return null
unexpectedly and could end up in NPEs resulting that did not before.
This commit addresses that situation by always ensuring a TravelAgent
instance is returned.

The TravelAgent for world 0 is returned arbitrarily in an effort to
compensate for plugins that are implementation dependent and expect some
form of a TravelAgent to be accessible in the event at all times.

By: EdGruberman <ed@rjump.com>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftTravelAgent.java

@@ -9,12 +9,17 @@ import org.bukkit.TravelAgent;
 
 public class CraftTravelAgent extends PortalTravelAgent implements TravelAgent {
 
+    public static TravelAgent DEFAULT = null;
+
     private int searchRadius = 128;
     private int creationRadius = 16;
     private boolean canCreatePortal = true;
 
     public CraftTravelAgent(WorldServer worldserver) {
         super(worldserver);
+        if (DEFAULT == null && worldserver.dimension == 0) {
+            DEFAULT = (TravelAgent) worldserver.s();
+        }
     }
 
     public Location findOrCreate(Location target) {