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.

[BUKKIT-3541](https://bukkit.atlassian.net/browse/BUKKIT-3541)