Queries with no results return a null array

[sudo-gab]

Hello!

This library allows you to query via StatementBuilder. When you run a query that returns no results, the XYPage#getResults() returns null. On my side, this has only been confirmed to be true with InventoryServiceInterface#getAdUnitsByStatement(), however it would be right to assume it is true for other query implementations as well.

Returning null arrays is an anti-pattern. Can this be fixed to return empty arrays instead? This has already been mentioned in #214.