fix: corrected logic for rollout services

[pawels-optimizely]

Summary

• corrected logic for rollout services: don't populate experiment when variation is null
• that should be safe and won't break notification center since

if featureDecision.Source == FeatureTest {
		sourceInfo["experimentKey"] = featureDecision.Experiment.Key
		sourceInfo["variationKey"] = featureDecision.Variation.Key
	}

is the only logic accessing featureDecision.Experiment directly. And our change is happening on the rollout only