a8n: Ignore jobs with empty diff as completed in CampaignPlan status

Created by: mrnugget

I ran into this while testing.

When 2 CampaignJobs are created for two repositories and one CampaignJob finishes successfully but has an empty diff it was counted towards the "completed" count in the campaignPlan.status.

From an API consumer perspective, this can be confusing since it means that this might be true:

campaignPlan.changesets.totalCount != campaignPlan.status.completedCount

Since we already filter out changesets that have finished but have no diff, with this change the following will always be true:

campaignPlan.changesets.totalCount == campaignPlan.status.completedCount