Check that comment isn't empty in TeamLabelInference#applicable?

What does this MR do and why?

I noticed that we recorded processor usage with duration: 0 in Clickhouse and I realized that Triage::TeamLabelInference would return a nil result if the comment to be posted would be nil or empty.

Triage::Telemetry.record_processor_execution would record the event even if result was nil, thus we ended up with more events in Clickhouse (82 rows in my example) than in the service logs (9 rows in my example):

Clickhouse	Service logs

This MR fixes both problems:

• Check the nil/empty message in Triage::TeamLabelInference#applicable? directly
• Don't send telemetry when result is nil so that it's consistent with the cases when we log an event

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review