Gracefully handle invalid team labels in `teams.json` when auto-creating package upgrade tickets

[mcmire]

When a new major version of a package is released, a workflow kicks off to create tickets in metamask-extension and metamask-mobile automatically. Tickets will get automatically assigned a label based on the mapping in teams.json.

However, if any of the labels in teams.json is invalid, then creating a ticket will fail. The workflow should create the ticket anyway but without a label, and then we can assign one manually.