Automatically use inner joins when filtering

Hi, first of all, thanks for building Cube.js and releasing it as open source, it's very cool software.

**Is your feature request related to a problem? Please describe.**
If you filter on a field in a way that precludes it from being `NULL`, then there is no functional difference between an inner and a left join on tables in that field. However, there are slight performance benefits to having an inner join here.

**Describe the solution you'd like**
When joins are added in order to support a filter, or really whenever a joined cube is filtered on, the generated join can be an `INNER JOIN` rather than `LEFT JOIN`. I think functionality that handles this case would be appreciated, in lieu of broader capability to influence join type and path.

This probably isn't super important / high priority, I'm just interested whether this in the roadmap. I can also have a go at doing it in a PR if somebody can direct me in the general direction of the relevant code.

Thanks again for your time and expertise!