Fix cannot remove block currency with mvm set command.

Setting currency to 0 is now considered as no currency value.

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

@@ -22,6 +22,8 @@ public class EntryFee extends SerializationConfig {
     @Nullable
     private Material currency;
 
+    private final Material DISABLED_MATERIAL = Material.AIR;
+
     public EntryFee() {
         super();
     }
@@ -51,6 +53,9 @@ public class EntryFee extends SerializationConfig {
      */
     @Nullable
     public Material getCurrency() {
+        if (currency == null || currency.equals(DISABLED_MATERIAL)) {
+            return null;
+        }
         return currency;
     }
 

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

@@ -50,6 +50,10 @@ public class TestEntryFeeConversion {
         WorldProperties props = new WorldProperties(config);
         assertNull(props.entryfee.getCurrency());
 
+        entryFee.put("currency", 0);
+        props = new WorldProperties(config);
+        assertNull(props.entryfee.getCurrency());
+
         entryFee.put("currency", 1);
         props = new WorldProperties(config);
         assertEquals(Material.STONE, props.entryfee.getCurrency());