Fix zookeeper annotations

[mouchar]

Fixed the case when no ZK annotations are set and zookeeper.restartPodsOnConfigMapChange: true helm can not render template

Motivation

When you don't set any annotations to Zookeeper (they are empty by default) AND if you set

zookeeper:
  restartPodsOnConfigMapChange: true

in your values.yaml file, the manifest can not be rendered. This commit fixes this condition and also unifies the
the way how the custom annotations are added, with bookkeeper statefulset template.

helm template pulsar apache/pulsar --version 3.0.0 --set zookeeper.restartPodsOnConfigMapChange=true  
Error: YAML parse error on pulsar/templates/zookeeper-statefulset.yaml: error converting YAML to JSON: yaml: line 52: did not find expected key

Use --debug flag to render out invalid YAML

Modifications

wrapping toYaml filter (that can return empty object {}) into with context.

Verifying this change

• Make sure that the change passes the CI checks.

[mouchar]

ping - any feedback?
Sorry, but I don't know how to "Make sure that the change passes the CI checks." when the Kind cluster doesn't start at all.