Removing from an empty JsonObjectBuilder may produce a NPE

[apyjz-gh]

It seems that builder does not initialize the valueMap field in all cases.

    public class Test {
        public static void main(String[] args) {
            JsonObjectBuilder builder = Json.createObjectBuilder();
            builder.remove("name");
        }
    }

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.Map.remove(Object)" because "this.valueMap" is null
at org.eclipse.parsson.JsonObjectBuilderImpl.remove(JsonObjectBuilderImpl.java:171)