[keyserver] Fix updateUserAvatar query

Summary:
We want to match the check introduced in D7430 (and fixed in D7435...) before setting container for any previous image avatar upload to NULL.

(Kind of) depends on D7435

Test Plan:

1. Set image avatar
2. Set emoji avatar
3. Set image avatar
4. Set malformed image avatar (uploadID doesn't point to upload that can be set as image avatar... whether because it's nonsensical or belongs to another uploader or is already contained by eg a message)

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7437