Skip updating if current retrospective doesn't exist

Skip updating if current retrospective doesn't exist

We can spot that some of the current retrospective issues cannot be found via the following log: https://gitlab.com/gitlab-org/async-retrospectives/-/jobs/2345284628

Issue http://does.not/exist to update

That caused the following actions to update the issue fail:

PUT to https://gitlab.com/api/v4/projects/gl-retrospectives%2Fmanage-stage%2fauthentication-and-authorization/issues/-1 failed!
Response code: 404. Body:
{"message":"404 Not found"}

Not sure why those don't exist (there were multiple), but in this case we probably don't want to update them.

Slack context: https://gitlab.slack.com/archives/CMA7DQJRX/p1650277284875619