Query DSL: Empty bool {} should return match_all

[ppf2]

This is somewhat related to the following request:
#6722

6722 actually causes a NPE when the clauses within the bool are null:

"bool" : {
    "must": [],
    "must_not": [],
    "should": []
  }

For this ticket, there are use cases when Kibana is generating requests like the following:

 "facet_filter": {
                "fquery": {
                    "query": {
                        "filtered": {
                            "query": {
                                "bool": {
                                }
                            },
                            "filter": {
                                "fquery": {
                                    "query": {
                                        "query_string": {
                                            "query": "_type:apache"
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }

The above query ignores the facet_filter's filter clause when it should really be returning a match_all plus the filter applied.

When a query with just a bool {} is run on its own, the empty bool clause in this case does not throw a NPE and is treated as a valid query, except that it returns no documents (when it should really be returning a match_all):

   "query": {
         "bool": {
         }
    }

[clintongormley]

An empty bool filter or query should be treated as a match_all.

[brwe]

Fixed, but I did not push to 1.2 because this relies on a change that is also not on 1.2 (d414d89). Let me know if you need it on 1.2 as well.