Avoid computing the next recurrence interval based on an invalid time

When Qt is build with asserts enabled this will abort otherwise, as seen in the FreeBSD CI.

In the unit tests we seem to hit this for recurring todos without a starting point, which doesn't seem entirely invalid. In those cases take our best guess for a starting point and continue from there, which for things like daily recurring tasks should be fine.

Alternative attempts with rejecting such recurrences outright break tests, so I assume that is not an option.