Removing from an empty JsonObjectBuilder may produce a NPE

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)