Add support for FilterMaintenanceAccessory

[#124](#124)

Author: gjvanderheiden

src/main/java/io/github/hapjava/accessories/optionalcharacteristic/AccessoryWithResetFilterIndication.java

@@ -9,6 +9,8 @@ public interface AccessoryWithResetFilterIndication {
    * Request to reset the filter level
    *
    * @param indication always 1, by HomeKit protocol. (to be ignored)
+   * @return a future that completes when the change is made
+   * @throws Exception when the change cannot be made
    */
   CompletableFuture<Void> resetFilterIndication(Integer indication) throws Exception;
 }

src/main/java/io/github/hapjava/characteristics/impl/base/IntegerCharacteristic.java

@@ -72,11 +72,7 @@ protected CompletableFuture<JsonObjectBuilder> makeBuilder(int iid) {
 
   @Override
   protected CompletableFuture<Integer> getValue() {
-    if (getter.isPresent()) {
-      return getter.map(integerGetter -> integerGetter.get()).get();
-    } else {
-      return null;
-    }
+    return getter.map(integerGetter -> integerGetter.get()).orElse(null);
   }
 
   @Override