backend: Add migration patch to bring existing users up to DB agnostic changes (#517)
* Modify past migrations to make DB agnostic
* add nullable value
* wip
* Add tested patch for existing users
* remove imports
* fixes
* lint
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Co-authored-by: Tianjing Li <tianjinglimail@gmail.com>
Signed-off-by: BeatrixCohere <128378696+BeatrixCohere@users.noreply.github.com>