Fixes Filter and FiltersAggregation to work with empty query

[colings86]

This fix ensures the filter and filters aggregation will not throw a NPE when {} is passed in as a filter. Instead {} is interpreted as a MatchAllDocsQuery.

Closes #17518

[javanna]

LGTM

[Mpdreamz]

This might be a feature not a bug but 6.0.0-alpha1 started throwing on this again:

{
  "aggs": {
    "empty_filter": {
      "filter": {}
    }
  }
}

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "query malformed, empty clause found at [4:18]"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "query malformed, empty clause found at [4:18]"
  },
  "status" : 400
}

[javanna]

@Mpdreamz it is indeed a feature :) See #22092 . Support for empty queries was deprecated in 5.0 and removed in 6.0.