Consider the "LIMIT 1" optimization with parallel DISTINCT

Consider the "LIMIT 1" optimization with parallel DISTINCT

Similar to what was done in 5543677ec for non-parallel DISTINCT, apply
the same optimization when the distinct_pathkeys are empty for the
partial paths too.

This can be faster than the non-parallel version when the first row
matching the WHERE clause of the query takes a while to find.  Parallel
workers could speed that process up considerably.

Author: Richard Guo
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAMbWs49JC0qvfUbzs-TVzgMpSSBiMJ_6sN=BaA9iohBgYkr=LA@mail.gmail.com