Fixed bug with facets when applying an empty facet filter

Hi Alexander,

I've fixed a minor bug related to facet filters. I found a case where an unwanted comma was outputted when the facet filter value was empty.

Before my fix:
"facets": { "Terms": { "terms": { "field": "StringProperty" }, } }

After my fix:
"facets": { "Terms": { "terms": { "field": "StringProperty" } } }

Thanks,
Mike