fix(people): fix searching by only campaign

[bchrobot]

Description

This PR does two things:

1. Fixes a BAD_USER_INPUT Apollo error
2. Fixes the join on assignment to work when searching only on campaign, and not user

Motivation and Context

Searching by campaign expected the user table to have already been joined but that was not necessarily the case.

How Has This Been Tested?

This has been tested locally.

Screenshots (if appropriate):

N/A

Documentation Changes

N/A

Checklist:

• My change requires a change to the documentation.
• I have included updates for the documentation accordingly.