fix: nil value handling in tplvalues.render helper

[Copilot]

When image.digest is undefined in values, templates render image:tag@null instead of image:tag. The application.tplvalues.render helper converts nil to string "null" via toYaml, which passes {{- with }} checks since "null" is non-empty.

Changes

• _helpers.tpl: Updated application.tplvalues.render to check for nil/invalid values before processing and return empty string instead of converting to "null"
• Test files: Added regression tests for cronjob, job, and deployment templates

Technical Details

The helper now short-circuits on nil/invalid values:

{{- define "application.tplvalues.render" -}}
    {{- $value := .value -}}
    {{- if or (not $value) (kindIs "invalid" $value) -}}
        {{- "" -}}
    {{- else if typeIs "string" $value -}}
        {{- tpl $value .context -}}
    {{- else -}}
        {{- tpl ($value | toYaml) .context -}}
    {{- end -}}
{{- end -}}

This fix benefits all uses of tplvalues.render throughout the chart, not just image handling. The {{- with }} checks now behave as expected for undefined values.

Before: nginx:latest@null
After: nginx:latest

Original prompt

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This section details on the original issue you should resolve

<issue_title>Image digest @null in rendered for cronjobs when not set</issue_title>
<issue_description>Describe the bug
When enabling cronjob, if a job has image.digest is not set the template will render @null.

To Reproduce
Steps to reproduce the behavior:

cronJob:
  enabled: true
  jobs:
    some-job:
      schedule: "* * * * *"
      image:
        repository: image-name
        tag: image-tag

Running helm template stakater/application -f values.yaml

# Source: application/templates/cronjob.yaml
apiVersion: batch/v1beta1
kind: CronJob
spec:
  schedule: "* * * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: some-job
            image: image-name:image-tag@null

A workaround is to explicitly set digest: ""

Affects 6.14 but I didn't see any mentions in recent releases.

Expected behavior
if digest is not set, it should not display.

Actual behavior
Digest displays with @null
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Image digest @null in rendered for cronjobs when not set #485

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