Harden for multiplayer

[Cervator]

At present a server-client will run weather fine, but if a solo client tries to join you crash with the following error:

20:46:41.010 [main] ERROR o.terasology.engine.TerasologyEngine - Uncaught exception, attempting clean game shutdown
java.lang.NullPointerException: null
	at org.terasology.weatherManager.systems.WeatherManagerSystem.postBegin(WeatherManagerSystem.java:127)
	at org.terasology.engine.modes.loadProcesses.PostBeginSystems.step(PostBeginSystems.java:51)
	at org.terasology.engine.modes.StateLoading.update(StateLoading.java:243)
	at org.terasology.engine.TerasologyEngine.tick(TerasologyEngine.java:458)
	at org.terasology.engine.TerasologyEngine.mainLoop(TerasologyEngine.java:421)
	at org.terasology.engine.TerasologyEngine.run(TerasologyEngine.java:397)
	at org.terasology.engine.Terasology.main(Terasology.java:156)
20:46:41.010 [main] INFO  o.terasology.engine.TerasologyEngine - Shutting down Terasology...

Probably a fairly typical multiplayer setup issue, not expecting it to be overly mysterious.

[Cervator]

A fix for this is pending in PR #9