Use empty list instead of nil when adding services (removes unnecessary churn)

[42wim]

This fixes an issue introduced by previous PR #3845

When doing the service checks for services without Service Tags we get []string{} back from master, but the service was locally created with []string(nil) which is again a difference and causes services (without service tags) to get synced again every ~2 minutes

cc: @slackpad

[slackpad]

Thanks @42wim for this fix! Churn without tags is probably worse than churn with tags, so I think we are going to cut a 1.0.5 release tomorrow with this.

While I was testing I noticed that node-level checks also have churn now as well, so I'm going to expand the unit test coverage and try to nail all the combinations so we know we've got a complete fix for this.

[slackpad]

Closing this in favor of #3867.