Aggragations/Filter - fix for es1.2 when no sub aggregations

Hi,

Small fix for Elastica/Aggregation/Filter.php. There is a bug when no subaggregation is set ... see "by_category_feature"
```json
"aggs": {
        "global": {
            "global": {},
            "aggs": {
                "by_category_feature": {
                    "filter": {
                        "match_all": {}
                    },
                  "aggs": []
                }
            }
        }
    }
```
This will cause an error in elasticsearch because "aggs" cannot be [] ... it should be {} .. or we can just omit the aggs when there are none present.

```json
"aggs": {
        "global": {
            "global": {},
            "aggs": {
                "by_category_feature": {
                    "filter": {
                        "match_all": {}
                    }
                }
            }
        }
    }
```

This will work fine :)