Allow filter aggregation without nested aggregations

[sventendo]

With is useful for getting only the basic doc_count value.

[sventendo]

You can have a complex filter for the aggregation but NOT care about aggregations within those filtered document and ONLY want a basic doc_count. But "aggs" can't be an empty array. This PR omits the "aggs" key altogether (like already done in "TermAggregation").

[AlexVanderbist]

Another great PR! thanks :)