Optimize FilterExec when output projection is empty

consider `select count(*) from t where id > 5`. I think we still build a mask, etc. internally but in the case we have no output projection we could maybe track just the number of matching rows as an integer? maybe there's already something in place, just jotting down thoughts.

Begin at FilterExec and reproduce the example query, `select count(*) from t where id > 5`. Inspect whether an empty output projection still builds a mask or other projection state; the issue names no files or tests, so completion criteria must include validating the count-only execution path and its performance.

Written by the indexing model from the issue text.