Guard against nothing in ThingGroupPicker.

This commit filters the result list of a ThingGroupPicker by a non-null
predicate to avoid null values in the resulting ThingGroup instance.

Since null values represent `nothing`, and we don't usually announce it
when players earn a `nothing` reward, it makes sense that they wouldn't
bubble up and somehow "manifest" in groups of things either.

Fixes #691

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

@@ -1,6 +1,7 @@
 package com.garbagemule.MobArena.things;
 
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Collectors;
 
 public class ThingGroupPicker implements ThingPicker {
@@ -15,6 +16,7 @@ public class ThingGroupPicker implements ThingPicker {
     public Thing pick() {
         List<Thing> things = pickers.stream()
             .map(ThingPicker::pick)
+            .filter(Objects::nonNull)
             .collect(Collectors.toList());
 
         return new ThingGroup(things);