[bitnami/gitea] Update externaldb-secret logic

[speedythesnail]

Description of the change

Changes the logic in the creation of the externaldb-secrets.yaml, so the secret is not created if you are specifying an exisitng secret.

Benefits

An unused blank-secret will not be created, contributing to reducing junk files.

Possible drawbacks

None that I know of.

Applicable issues

• fixes #27595

Additional information

The chart is using the specified externaldb secret and key in the values.yaml, however it still creates an externaldb-secret, with no password. This fixes that.

Checklist

• [X] Chart version bumped in Chart.yaml according to semver. This is not necessary when the changes only affect README.md files.
• [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)