Fix a potential Diesel deserialization crash

• Filter out empty UUID strings

Draft while I'm working with someone on an issue introduced in beta 18

2023-06-01 11:39:50.474 [0m[34m[DEBUG] [0mlibsignal_service_actix::push_service: HTTP request GET https://chat.signal.org/v1/accounts/whoami
2023-06-01 11:39:50.476 [0m[34m[DEBUG] [0mlibsignal_service_actix::push_service: HTTP request GET https://chat.signal.org/v2/keys?identity=aci
2023-06-01 11:39:50.476 [0m[TRACE] [0mlibsignal_service_actix::websocket: Will start websocket at Url { scheme: "wss", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("chat.signal.org")), port: None, path: "/v1/websocket/", query: Some("xxx"), fragment: None }
2023-06-01 11:39:50.480 [0m[TRACE] [0mwhisperfish_store::store: Called fetch_recipient_by_id(1)
2023-06-01 11:39:50.481 [0m[32m[INFO] [0mOur own profile is up-to-date, not fetching.
2023-06-01 11:39:50.481 [0m[TRACE] [0mwhisperfish_store::store::migrations::session_to_db: calling migrate_identities
fatal runtime error: failed to initiate panic, error 9
Redirecting call to abort() to mozalloc_abort

I had a similar crash while debugging !421 (merged)