fix(lifecycle): filter out personless events from lifecycle insights

[thmsobrmlr]

Problem

Anonymous persons show up as resurrecting in lifecycle insights. This is because we look at the persons’ created at (events.person.created_at) to determine wether they are new or resurrecting. Since anonymous persons don't have a person profile, this property is null which then looks like the person has been there since the beginning of (unix) time.

Closes: #25367

Changes

This PR filter out personless events in the lifecycle insight.

An alternative approach would be to look at the first event associated with each person_id for anonymous users. However, I don’t see a performant way to achieve this without creating some form of a person profile. For thoroughness, I attempted to join an events query without date limits, but this exceeded the memory limit for our team.

tbd: how do we signal to users that this insight excludes anonymous events, while e.g. retention has them? just docs?

How did you test this code?

Added a test and manual testing

before:

after: