Resolve "Repose | Fix query run with no symptom checks"

Closes #922 (closed)

Test by removing ReposePeriodicSymptomCheckStoreModule and ReposeSporadicSymptomCheckStoreModule from features.module and emptying IndexedDB.

The 1st commit fixed the issue: Clicking "Fragen beantworten" when no SCs are due redirects to the home screen and the empty SC selection modal informs the user that there aren't any SCs. IMO this is better than silently redirecting to home, which may give the user the impression that the button doesn't work.

The 2nd commit tried to fix a related issue (cf #928) where the ugly empty query run can still be accessed by clicking outside of the SC selection modal. #928 fixed the close button, but didn't take into account clicking outside of the modal. I'm sure my solution can be improved, but I didn't know how to catch "clicking outside of the modal". Guidance appreciated 👐

Update: Andreas advised to leave the 2nd point as it is, cf #951