fix: issue where helm index.yaml cannot be read (#13127)

* Fix issue where helm index.yaml cannot be read

The result of this function call is destructured here: https://github.com/renovatebot/renovate/blob/d0d482155dbc8ec6d901105c246fb1542b6a3b00/lib/datasource/helm/index.ts#L65



If the return value is undefiend this causes an error, an empty object satisfies the RepoSource interface and can also be destructured without issue.

* update tests to match helm findSourceUrl behaviour

Co-authored-by: Michael Kriese <michael.kriese@visualon.de>