[bitnami/external-dns] add default pihole.password value

[jamesharr]

Description of the change

This change adds a default value for pihole.password. When using a provider other than pihole, such as AWS, the secret.yaml template would fail to render with the following error:

Error: UPGRADE FAILED: template: external-dns/templates/secret.yaml:108:16: executing "external-dns/templates/secret.yaml" at <.Values.pihole.password>: nil pointer evaluating interface {}.password

Benefits

Does not force users who want a different provider to have to configure a pihole password

Possible drawbacks

Applicable issues

Additional information

Checklist

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

[jamesharr]

No worries @migruiz4, we all have other stuff we're working on.