[bitnami/harbor] Fix REGISTRY_HTTP_SECRET creation when not using an existing secret

[uivraeus]

Description of the change

Ensure that REGISTRY_HTTP_SECRET can be assigned when there is no existing Secret with a value for it.

The fix is to let the temporary variable used for a potentially existing secret value default to an empty string. This follows the implementation strategy applied in "core-secret-envvars.yaml"

Benefits

Possible to control the value of REGISTRY_HTTP_SECRET

Possible drawbacks

None

Applicable issues

• fixes [bitnami/harbor] Invalid logic prevents assignment or auto-generation of REGISTRY_HTTP_SECRET in harbor-registry secret #31008

Additional information

None

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)