JsonArray serialization produced an incorrect result

The serialized result is missing a comma.

```java
        System.out.println(Jsonb.builder().build().type(JsonArray.class)
                .toJson(
                        JsonArray.create().add(JsonArray.create().add("")).add(JsonArray.create().add(""))
                )
        );
```

Expected result  
```
[[""],[""]]
```
Actual result
```
[[""][""]]
```