fix(NightJson): correctly save empty maps

src/main/java/eu/midnightdust/yaytris/util/NightJson.java

@@ -90,7 +90,7 @@ public class NightJson {
             StringBuilder mapPairs = new StringBuilder();
             Map<?, ?> map = (Map<?, ?>) value;
             Iterator<?> it = map.keySet().iterator();
-            if (it.hasNext()) mapPairs.append("{");
+            mapPairs.append(it.hasNext() ? "{" : "{}");
             while (it.hasNext()) {
                 Object key = it.next();
                 Object val = map.get(key);