(core) Skip member-limit check for the merged pseudo-org

Summary:
Previously, if a member cap (maxUsersPerOrg) ever applied to a personal
org's plan, getUserLimitStatus() would try to get a user count for org
ID of 0, and fail with "_org: requires userId".

Test Plan: Test case added.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D5065