DATAMONGO-829 - NearQuery should not default 'num' to zero.

`NearQuery` now ignores `query.getLimit()` equal to zero, when adding `Query` to `NearQuery`. This has to be done as `limit` is defaulted to zero within `Query` which then results in unintended propagation of the parameter.

In case `num` should be explicitly set to `zero` one might use `NearQuery.num(0)` as an alternative to the query approach.

Introduced `null` check for `NearQuery.query(Query)` and `NearQuery.with(Pageable)` along the way.