Add explicit initialization for all PlannerGlobal fields

Add explicit initialization for all PlannerGlobal fields

When creating a new PlannerGlobal node in standard_planner(), most
fields are explicitly initialized, but a few are not.  This doesn't
cause any functional issues, as makeNode() zeroes all fields by
default.  However, the inconsistency is undesirable from a clarity and
maintenance perspective.

This patch explicitly initializes the remaining fields to improve
consistency and readability.

Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAMbWs4-TgQHNOiouqGcuHoBqbJjWyx4UxGKxUY3FrF4trGbcPA@mail.gmail.com