This saves a few lines of code.  Also add a comment to mention why we use
ExplainPropertyInteger instead of ExplainPropertyUInteger given that
queryid is a uint64 type.

Author: David Rowley
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAApHDvqhSLYpSU_EqUdN39w9Uvb8ogmHV7_3YhJ0S3aScGBjsg@mail.gmail.com
Backpatch-through: 14, where this code was originally added