Prometheus config rendered from struct no longer parses

[tcolgate]

5c89cec breaks configs rendered from the internal config struct (yeah, I know it's nasty, but we build prom configs automatically and we vendor prom to support it).

The above change beaks because the config renders as

...
relabel_rules:
  - action: labeldrop
    source_labels: []
...

either source_labels should include "omitempty", or the above sourcelabel check should be updated to something which parses with nil vs empty slice (e.g. len())

What did you do?

Attempted to run prometheus master with a configuration from our config management tool.

What did you expect to see?

Prometheus accept the configuration and start.

What did you see instead? Under which circumstances?

prometheus fails with ```time="2017-07-04T12:10:18Z" level=error msg="Error loading config: couldn't load configuration (-config.file=/etc/prometheus/config.yaml): labeldrop action requires only 'regex', and no other fields" source="main.go:160"

**Environment**

Linux 

* System information:

	Linux 4.7.0-0.bpo.1-amd64 x86_64

* Prometheus version: 

	image from quay.io master

[lock]

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