Add support for picking nothing.

This commit introduces the strange concept of a singleton ThingPicker
that only ever picks `null`. The purpose of this picker is to allow for
a type of "loot table" experience similar to that found in other games.
An example would be a piece of equipment that only has a 50 % chance of
dropping. With the current state of MobArena, it would be necessary to
something conjure up a CommandThing or something to emulate nothingness,
but now there is native support for it with the `nothing` keyword.

The nullability of rewards also has the side effect that we got to clean
up the MASpawnThread `addReward` method a bit.

Closes #638

src/main/java/com/garbagemule/MobArena/MASpawnThread.java

@@ -392,13 +392,8 @@ public class MASpawnThread implements Runnable
     private void addReward(ThingPicker picker) {
         for (Player p : arena.getPlayersInArena()) {
             Thing reward = picker.pick();
-            rewardManager.addReward(p, reward);
-
-            if (reward == null) {
-                arena.getMessenger().tell(p, "ERROR! Problem with rewards. Notify server host!");
-                plugin.getLogger().warning("Could not add null reward. Please check the config-file!");
-            }
-            else {
+            if (reward != null) {
+                rewardManager.addReward(p, reward);
                 arena.getMessenger().tell(p, Msg.WAVE_REWARD, reward.toString());
             }
         }

src/main/java/com/garbagemule/MobArena/MobArena.java

@@ -16,6 +16,7 @@ import com.garbagemule.MobArena.metrics.VaultChart;
 import com.garbagemule.MobArena.signs.ArenaSign;
 import com.garbagemule.MobArena.signs.SignBootstrap;
 import com.garbagemule.MobArena.signs.SignListeners;
+import com.garbagemule.MobArena.things.NothingPickerParser;
 import com.garbagemule.MobArena.things.RandomThingPickerParser;
 import com.garbagemule.MobArena.things.ThingGroupPickerParser;
 import com.garbagemule.MobArena.things.ThingManager;
@@ -71,6 +72,7 @@ public class MobArena extends JavaPlugin
         pickman = new ThingPickerManager(thingman);
         pickman.register(new ThingGroupPickerParser(pickman));
         pickman.register(new RandomThingPickerParser(pickman, random));
+        pickman.register(new NothingPickerParser());
     }
 
     public void onEnable() {

src/main/java/com/garbagemule/MobArena/things/NothingPicker.java

@@ -0,0 +1,21 @@
+package com.garbagemule.MobArena.things;
+
+class NothingPicker implements ThingPicker {
+
+    private static final NothingPicker instance = new NothingPicker();
+
+    @Override
+    public Thing pick() {
+        return null;
+    }
+
+    @Override
+    public String toString() {
+        return "nothing";
+    }
+
+    public static NothingPicker getInstance() {
+        return instance;
+    }
+
+}

src/main/java/com/garbagemule/MobArena/things/NothingPickerParser.java

@@ -0,0 +1,13 @@
+package com.garbagemule.MobArena.things;
+
+public class NothingPickerParser implements ThingPickerParser {
+
+    @Override
+    public ThingPicker parse(String s) {
+        if (!s.equalsIgnoreCase("nothing")) {
+            return null;
+        }
+        return NothingPicker.getInstance();
+    }
+
+}