fix(datanode): use with for each secret field

[monrax]

Summary

Small refactor: prevent empty fields from being rendered in DN secrets.

What changed

• Use with in template for S3 fields in secret/datanode.yaml

Linked issues

This closes #68

Checklist

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

Installation

• Fresh installation completes successfully: helm install graylog ./charts/graylog
• All pods reach Running state: kubectl rollout status statefulset/graylog
• Helm tests pass: helm test graylog

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Cluster Configuration
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

Specific to this PR

• Verify no fields are set for a regular installation:

  # install normally
  helm install graylog ./charts/graylog -n graylog --create-namespace
  
  # decode the graylog-secrets-datanode secret
  kubectl get secret graylog-secrets-datanode -n graylog -o jsonpath='{.data}'  | jq 'map_values(@base64d)'

Notes for reviewers

• Verify all tests above pass
• Sync up with the author before merging
• The commit history should be preserved - use rebase-merge or standard merge options when applicable