[Bleeding] Don't allow null display names. Fixes BUKKIT-5254

Currently, plugins can set a player's display name to null, which could
cause issues for other calls to getDisplayName that aren't expecting a null
value. This changes setDisplayName to follow the same logic as
setPlayerListName, which sets the player's name back to their unmodified
"vanilla" name if it receives a null value as a parameter.

src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java

@@ -169,7 +169,7 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
 
     @Override
     public void setDisplayName(final String name) {
-        getHandle().displayName = name;
+        getHandle().displayName = name == null ? getName() : name;
     }
 
     @Override