fix: Improve external Redis validation in Argo CD chart

[jamesmr89]

The current validation for external Redis in the Argo CD Helm chart is triggering false positives when Redis is disabled and an external Redis is configured. This commit updates the validation logic to properly check for the presence of external Redis configuration when the built-in Redis is disabled.

Changes:

• Update the argocd.validateValues.externalRedis function in _helpers.tpl
• Add separate checks for external Redis host and port
• Provide more specific error messages for missing configuration

This change ensures that users can successfully deploy Argo CD with an external Redis configuration without encountering false validation errors. It maintains backwards compatibility while improving the user experience for those using external Redis instances.

Description of the change

Benefits

Possible drawbacks

Applicable issues

• fixes #

Additional information

Checklist

• Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• Variables are documented in the values.yaml and added to the README.md using readme-generator-for-helm
• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)

[carrodher]

Thanks for your contribution! Please note it's needed to bump the chart version in the Chart.yaml