[prometheus-blackbox-exporter] fix issue when set extra containers

[allanhung]

What this PR does / why we need it

fix issue when set extra containers

cat > values.yaml << EOF
extraInitContainers:
- name: k8s-sidecar
  image: quay.io/kiwigrid/k8s-sidecar
extraContainers:
- name: k8s-sidecar
  image: quay.io/kiwigrid/k8s-sidecar
EOF
helm template -f values.yaml prometheus-community/prometheus-blackbox-exporter
Error: YAML parse error on prometheus-blackbox-exporter/templates/deployment.yaml: error converting YAML to JSON: yaml: line 37: block sequence entries are not allowed in this context

Use --debug flag to render out invalid YAML

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes #

Special notes for your reviewer

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])