Don't render empty groups #36

Is your feature request related to a problem? Please describe.
Currently creating an empty group using .and(q -> q) or .or(q -> q) will result in an empty expression group which will make the query fail.

Describe the solution you'd like
Instead of producing invalid sql the empty group (and the preceding conjunction) shouldn't be rendered.

Describe alternatives you've considered
An alternative would be to produce an always false expression but that's probably not what somebody would expect and makes building optional clauses way harder.