feat(helm): Make Jena passwords optional in values file.

[aledegano]

This commit continues on the topic of making the definition of the secrets in the values files optional, to simplify CI deployments.

As previous similar commits this change supports:

• Existing deployments where the secret is defined will keep working as usual

• New deployments where the secret is defined will keep working as they did before this change

• New deployments where the secret is NOT defined will get 2 random passwords which will be persisted across upgrades/redeployments

The three scenarios have been manually tested.

This PR doesn't change anything to users or admins, therefore I didn't write an entry in the changelog to avoid unnecessary noise, let me know if you would rather have one.