use Collections' factory method instead of static field

this resolves some unchecked generics warning

src/main/java/com/onarandombox/MultiverseCore/MVWorld.java

@@ -356,7 +356,7 @@ public class MVWorld implements MultiverseWorld {
 
         @Override
         public Map<String, Object> serialize() {
-            return Collections.EMPTY_MAP;
+            return Collections.emptyMap();
         }
 
         /**

src/main/java/com/onarandombox/MultiverseCore/MultiverseCore.java

@@ -534,7 +534,7 @@ public class MultiverseCore extends JavaPlugin implements MVPlugin, Core {
             } else if (entry.getValue() instanceof ConfigurationSection) {
                 Logging.fine("Migrating: " + entry.getKey());
                 // we have to migrate this
-                WorldProperties world = new WorldProperties(Collections.EMPTY_MAP);
+                WorldProperties world = new WorldProperties(Collections.emptyMap());
                 ConfigurationSection section = (ConfigurationSection) entry.getValue();
 
                 // migrate animals and monsters

src/test/java/com/onarandombox/MultiverseCore/TestEntitySpawnRules.java

@@ -101,42 +101,42 @@ public class TestEntitySpawnRules {
     @Test
     public void test() {
         // test 1: no spawning at all allowed
-        adjustSettings(false, false, Collections.EMPTY_LIST, Collections.EMPTY_LIST);
+        adjustSettings(false, false, Collections.emptyList(), Collections.emptyList());
         createAnimals();
         spawnAllNatural();
         verify(sheepEvent).setCancelled(true);
         verify(zombieEvent).setCancelled(true);
 
         // test 2: only monsters
-        adjustSettings(false, true, Collections.EMPTY_LIST, Collections.EMPTY_LIST);
+        adjustSettings(false, true, Collections.emptyList(), Collections.emptyList());
         createAnimals();
         spawnAllNatural();
         verify(sheepEvent).setCancelled(true);
         verify(zombieEvent).setCancelled(false);
 
         // test 3: all spawning allowed
-        adjustSettings(true, true, Collections.EMPTY_LIST, Collections.EMPTY_LIST);
+        adjustSettings(true, true, Collections.emptyList(), Collections.emptyList());
         createAnimals();
         spawnAllNatural();
         verify(sheepEvent).setCancelled(false);
         verify(zombieEvent).setCancelled(false);
 
         // test 4: no spawning with zombie exception
-        adjustSettings(false, false, Collections.EMPTY_LIST, Arrays.asList("ZOMBIE"));
+        adjustSettings(false, false, Collections.emptyList(), Arrays.asList("ZOMBIE"));
         createAnimals();
         spawnAllNatural();
         verify(sheepEvent).setCancelled(true);
         verify(zombieEvent).setCancelled(false);
 
         // test 5: all spawning with sheep exception
-        adjustSettings(true, true, Arrays.asList("SHEEP"), Collections.EMPTY_LIST);
+        adjustSettings(true, true, Arrays.asList("SHEEP"), Collections.emptyList());
         createAnimals();
         spawnAllNatural();
         verify(sheepEvent).setCancelled(true);
         verify(zombieEvent).setCancelled(false);
 
         // test 6: eggs
-        adjustSettings(false, false, Collections.EMPTY_LIST, Collections.EMPTY_LIST);
+        adjustSettings(false, false, Collections.emptyList(), Collections.emptyList());
         createAnimals();
         spawnAll(SpawnReason.SPAWNER_EGG);
         verify(sheepEvent, never()).setCancelled(anyBoolean());