The `sum()` and `count()` functions were giving results different from the openCypher specification on null `input.` The aggregation functions also had a problem when they were used in a group-by context and were giving results that were not compliant with the openCypher specification.
* Make `IfOperator` return the `else_expression_` in case of `NULL`
* Add gql_behave tests
* Add gql_behave test to specifically check for the case when the test expression itself is null
