[keyserver] Cast NULLs in UNION SELECT

Summary:
See comment on Linear. This fixes a bug where we were seeing strings where we previously saw numbers.

Depends on D4528

Test Plan:

1. I made sure the syntax was valid on both MySQL 5.7 and MariaDB 10.8 based on docs
2. I tested with MariaDB 10.8
3. I made sure the issue only occured when merging in NULL in cases where you aren't expecting a string by testing queries
4. I git grepd through the codebase to make sure this was the only place we were doing that

Reviewers: jonringer-comm, atul, palys-swm

Reviewed By: atul

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4530