Correctly handle outliers during persist events

[erikjohnston]

We incorrectly asserted that all contexts must have a non None state
group without consider outliers. This would usually be fine as the
assertion would never be hit, as there is a shortcut during persistence
if the forward extremities don't change.

However, if the outlier is being persisted with non-outlier events, the
function would be called and the assertion would be hit.

Fixes #3601

[richvdh]

lgtm