BUG #18869: /src/backend/optimizer/plan/createplan.c clause can be null if user isn't root

The following bug has been logged on the website:

Bug reference:      18869
Logged by:          Daniel Elishakov
Email address:      dan-eli@mail.ru
PostgreSQL version: 17.4
Operating system:   ubuntu 20.04
Description:

If root is NULL, pointer clause can also be NULL and then dereferenced at
createplan.c:5104 at if (IsA(clause, OpExpr)). I guess there should be a
check for NULL.