Do not run the window function computation step when the select
produces no rows (zero_result_cause!=NULL).
This may cause reads from uninitialized memory.

We still need to run the window function computation step when
the output includes just one row  (for example
SELECT MAX(col), RANK() OVER (...) FROM t1 WHERE 1=0).

This fix also resolves an issue with queries with window functions
producing an output row where should be none, like in
SELECT ROW_NUMBER() FROM t1 WHERE 1=0.

Updated a few test results in the existing tests to reflect this.