Currently we support empty query clauses like the filter in
"constant_score" : { "filter" : { } }
How these clauses are handled depends on the surrounding query.
They later are either ignored, converted to match all or no documents or
passed up further in the query hierarchy. During parsing these claues are
currently represented as EmptyQueryBuilders. When not handled anywhere else,
these special cases need to be checked for on the shard when building the
lucene query.
This is trappy, so this PR changes the parsing of compound queries. Instead
of returning QueryBuilder, the core query parsing method
QueryShardContext#parseInnerQueryBuilder() now return an Optional which can
be empty in the case of empty query clauses. This has the advantage of forcing
callers to deal with this sooner or later. When encountering empty Optionals,
compound query builders now have the choice to ignore them, pass them on or
rewrite to a different query, depending on context.
359f45988f