[web] avoid initializing already initialized WASM module

Summary: Context in ENG-6276.

Test Plan:

1. Open multiple tabs and make sure DB is initialized only once
2. Log out and make sure it works (in that case we have WASM module, but don't have SQLiteQueryExecutor)

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10439