No UI error on headless server on player death

[jdrueckert]

What happened

I died in-game and the server logged an error.

How to reproduce

1. Run a headless server
2. Join the server
3. Die
4. Check the server logs and see the following error logged:

   18:28:18.165 [main] ERROR o.t.e.network.internal.NetClient - Error processing event
   java.lang.NullPointerException: Cannot invoke "org.terasology.engine.rendering.nui.NUIManager.getHUD()" because "this.nuiManager" is null
       at org.terasology.module.inventory.systems.CharacterInventorySystem.resetDropMark(CharacterInventorySystem.java:212)
       at org.terasology.module.inventory.systems.CharacterInventorySystem.onPlayerDeath(CharacterInventorySystem.java:241)
   

Proposed Fix

A headless server shouldn't try to run the respective code path in the first place as it is not expected to have a UI.
If the respective code is relevant for players, this should be run client-side.