fix(loki): prevent empty postRenderer key when no post-renderers are enabled

General MR

Summary

This MR fixes a minor bug in the loki helm release template that causes it to render postRenderers as an empty key with certain configurations.

Relevant logs/screenshots

N/A (Added tests to verify)

Linked Issue

Closes #2914 (closed)

Relates to big-bang/pipeline-templates/pipeline-templates#784

Upgrade Notices

N/A