fix(sharing): Don't crash when loading shares for unknown user

[nickvergessen]

• Since feature: implement IPartialShareProvider support #2275 the share provider crashes when loading shares of a deleted user
• This is triggered via the new UserShareAccessUpdatedEvent even that apps are supposed to trigger when access changes.
• The event is dispatched by Talk when removing a user from the one-to-one chats and migrating them to former-one-to-one chats
• Failure visible before in https://github.com/nextcloud/spreed/actions/runs/20985168552/job/60317795918?pr=16713