Loki can create an empty postRenderers key under certain configurations

Motivation

When istio is enabled but loki is deployed in a mode other than distributed, it creates the postRenderers key with no content. This can cause issues when Big Bang is deployed as a flux HelmRelease with drift detection enabled:

bigbang     bigbang                      50m   False   Could not determine release state: unable to determine cluster state: HelmRelease/bigbang/loki dry-run failed (Invalid): HelmRelease.helm.toolkit.fluxcd.io "loki" is invalid: [spec.postRenderers: Invalid value: "null": spec.postRenderers in body must be of type array: "null", <nil>: Invalid value: "null": some validation rules were not checked because the object was invalid; correct the existing errors to complete validation]

Acceptance Criteria

• Loki's HR must not have a postRenderers key when no postRenderers are supplied