Short circuit if query yields no results

[hancush]

Thanks so much for this awesome library!

This PR adds a logic to short circuit further queries if the initial result count is 0. It also adds a test for this behavior.

I made this change because the min/max IDs of an empty result set are null (e.g.), which causes _get_layer_min_max to fail with the error reported here: datamade/census_area#12

I thought it made more sense to stop if we know there are no results to return, rather than to handle the error in the min/max comparison.

I like that this approach is agnostic about whether no results is good or bad (as opposed to raising an error), but I'm happy for your feedback, if you'd prefer a different behavior!

Let me know what you think. 🙃

[iandees]

Thanks for this improvement! It looks good. Thanks especially for adding a test for it.