fix(helm): Leave trailing newline in extraObjects

[vholer]

What this PR does / why we need it:

The change from af5be90 introduced an option use strings (prerendered/templated YAMLs) as an extraObjects in Helm values. This change breaks previous behaviour when extraObjects where list of dict for more than 1 object.

The currently rendered content of production/helm/loki/templates/extra-manifests.yaml is now broken in between of individual objects, because the trailing newline is stripped. Example such wrongly rendered document:

...
metadata:
  name: loki-rules-infra---
apiVersion: v1
data:
...

Obviously, the three-dashes document separator (---) should be on it's own line. Correctly should be:

...
metadata:
  name: loki-rules-infra
---
apiVersion: v1
data:
...

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR