Fix idler choosing wrong feature toggle impl

[sthaha]

Feature toggle has 2 implementations and the way it is chosen in main is based on the value of JC_FIXED_UUID environment variable. e.g.

// main.go: createFeatureToggle

if len(config.GetFixedUuids()) > 0 {
  f, err = toggles.NewFixedUUIDToggle(config.GetFixedUuids())
} else {
  f, err = toggles.NewUnleashToggle(config.GetToggleURL())
}

The recent change to idler configuration package to use viper ( 73d8f0ed6864d40905cd72747a517de59adababf ) introduced a change (regression) that resulted in GetFixedUuids returning a []string{ "" } when the variable isn't set.

This meant that idler would always choose the FixedUUID toggle instead of using the actual feature toggle service.

This patch fixes the issue by fixing GetFixedUuids to return empty slice if the env var isn't set.

Fixes #333