Address #1364 by adding emit_nil_on_no_rows setting

[mvitaly]

If this is the desired direction for resolving #1364 , I can further work on this PR and add documentation

[xeoncross]

This looks pretty good, @kyleconroy have you looked at this by chance?

[kyleconroy]

Please take a look at my comment here (https://github.com/kyleconroy/sqlc/issues/1364#issuecomment-1233613432). I'd like to hold off on merging this change until we've tried to solve it a few different ways.