Update path interpolate function to allow root

[bdeferme]

Pull Request (PR) description

The issue with the current interpolate function: it silently drops the root path (/). This happens because split('/') on / returns an empty array ([]), which is then skipped due to the unless segments.empty? condition.

For our use case, the root path (/) is a valid and meaningful entry. It allows us to use explicit lookups in hieradata, which brings several benefits:

• Reduced traversal through the vault tree.
• Improved readability of configurations.
• More flexibility in how we structure and migrate secrets.

For example, with a path list like:

• /
• nodes/%{facts.networking.fqdn}
• environments/%{trusted.extensions.pp_role}
• common

We can resolve secrets like this in common.yaml:

profile::cool::access_key: "%{lookup('provider/s3/access-keys/fancy-prd.key')}"

This would map to the vault path /provider/s3/access-keys/fancy-prd.key.

We’re migrating all our secrets to explicit lookups, and this path list allows us to do it gradually, rather than in a big-bang migration. However, the current implementation makes this difficult.

[yakatz]

This looks good - please fix the rubocop warnings.