Summary:
This fixes an issue when aggregations and/or group by expressions
weren't picked up from certain operators. In addition to that, we would
segfault in cases when the `has_aggregation_` is empty. For example,
function calls without arguments: `RETURN PI()`.
Test aggregations inside some operators
Reviewers: florijan, mislav.bradac, buda
Reviewed By: mislav.bradac
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D425