Gracefully handle non-existance of event_name when trying to get auto-events

[badboy]

Fixes #2310

Checking the public data this is based on this is (part of) what's currently in there:

[
  {
    "app": "accounts_frontend",
    "count": "1"
  },
  {
    "app": "accounts_frontend",
    "event_name": "glean.element_click.email_first_submit1DasO6R8')) OR 887=(SELECT 887 FROM PG_SLEEP(15))--",
    "count": "1"
  },

The first entry doesn't have any event_name (and others have some SQL injection tries there).
I don't actually know where we generate that file. We can probably fix it in the generation to not even generate that first entry, but regardless I think this dictionary ETL should be resistant against slightly wrong input data.

[edugfilho]

Thank you for the fix!