Do not initialize a synchronization run_at if interval is nil
CDB-673 Dyslexia
Merge remote-tracking branch 'origin/feature/CDB-673' into release/2.3.2
* Mixpanel Log Event: Data Imports
Merge remote-tracking branch 'origin/feature/CDB-818' into release/2.3.2
* Update run_at when changing synchronization interval of a table