Prevent PHP Warning when Statistics is null

[puntope]

Changes proposed in this Pull Request:

Closes part of #2398

This PR prevents a PHP warning when statistics is null as we try to access ['statistics']['active`] in some parts of the code without checking if it's set.

Detailed test instructions:

1. Enable debug logging to see PHP warnings
2. Clear statistics cache
3. Trigger the scheduled action to run notification ( wc_gla_cron_daily_notes )
4. Confirm that the PHP warning is not generated and note is not created

Notice that some conditions are required for this Note to be triggered. For testing this I recommend commenting off some checks under AbstractSetupCampaign::should_be_added or be sure you pass all those checks.

So we can focus on the problematic method $this->merchant_center_service->has_at_least_one_synced_product()

Additional details:

Changelog entry

Fix - Prevent PHP Warning when Statistics is null

[ianlin]

Thanks for fixing it, I no longer see the error message after running the job when statistics doesn't exist. Just one question, in the PR description you mentioned:

Closes part of #2398

Would it still be follow ups, or it already completely solved the issue #2398?