#4444 @swamp/video: rendering an edit list that keeps nothing fails with 'sumExpr needs at least one term'

Found during swamp-club#4330, and present on main. With two-pass loudness (the default), render's measuring pass builds the select expression before buildGraph runs, so a take whose edit list keeps nothing throws 'sumExpr needs at least one term' rather than buildGraph's 'the edit list keeps nothing: keep some of the take (edit.txt or the keep method) before rendering'. Reproduce: video-edit on a recording with no speech (a silent 8s clip does it). Joined renders are not affected, since checkJoin runs first. Expected: the friendly message, before any ffmpeg pass.