raw-configuration: fix invalid resources issued from comment-only YAML fragments

[LeonoreMangold]

Example of the error:

templates:
  - |
    # Create my priority class
    ---
    apiVersion: scheduling.k8s.io/v1
    kind: PriorityClass
    metadata:
      name: test
    value: 1
    preemptionPolicy: Never
    globalDefault: false
    description: "my first priority class"

Here the previous version of the chart would generate two Kubernetes manifests, the first one resulting from the comment part ("# Create my priority class") before the first object separator "---".
The first document only contained the labels appended by the chart, something like that:

---
metadata:
  labels:
    app: raw
    chart: raw-1.0.2
    heritage: Helm
    release: raw

But this is an invalid Kubernetes manifest since it does not contain mandatory fields like apiVersion or kind, so applying the Helm release will result in an error.

This kind of use case with a comment being placed before the first separator "---" can occur when using a range loop that needs to be documented. Like that:

{{- range $key, $value:= .Values.priorityClasses }}
# An explanatory comment about what's coming below
---
apiVersion: scheduling.k8s.io/v1
kind: PriorityClass
metadata:
  name: {{ $key }}
value: {{ $value.priority }}
preemptionPolicy: Never
globalDefault: false
description: {{ $value.desc }}
{{- end }}

[LeonoreMangold]

Note: converting to a yaml document removes the comments and only outputs the "real" manifest content. This should be the most reliable base to test for emptiness.