fix: make tls-secret-name optional for lego use

[mertalpt]

Description

The charm makes tls-secret-name a mandatory configuration variable. That particular variable is used with nginx-ingress-integrator to supply a TLS certificate manually. However, the Lego charm can automate acquiring TLS certificates. Unfortunately, tls-secret-name currently cannot be left blank and using a dummy variable causes the ingress to get a default certificate that does not work.

This PR make tls-secret-name optional. This particular change has been tested via hacking the charm code manually and worked.

Notes for Code Reviewers

The PR will be in draft until I can write proper tests. Unless we decide to forego tests for this change.

[mertalpt]

@kelkawi-a Should I update the libraries? Should I write new tests for the Lego integration? We can do that later™ too.

[mertalpt]

Given how long it takes to run tests, I am opting to merge now write tests later. The change is very small.

[kelkawi-a]

@kelkawi-a Should I update the libraries? Should I write new tests for the Lego integration? We can do that later™ too.

Library updates can be a separate PR. No need for Lego charm tests because that is integrated with the nginx-ingress-integrator charm, not directly to the Ranger charm.