Error upgrading mode: operator k8s charms

[David]

Summary: Cannot upgrade an operator charm in k8s without getting an error.

This is the metadata.yaml of the charm I am trying to deploy:

```yaml
name: kdu-proxy
description: |
  TEMPLATE-TODO: fill out the charm's description
summary: |
  TEMPLATE-TODO: fill out the charm's summary
series: [kubernetes]
deployment:
  mode: operator
```

The deployment:mode: operator means that there is not workload associated to the charm. When I upgrade the charm, I get the following error in the debug log:

```
ERROR juju.worker.caasoperator.uniter.kdu-proxy/0.operation hook "upgrade-charm" (via hook dispatching script: dispatch) failed: cannot apply changes: cannot set k8s spec on an operator charm
```

I think the problem is that in Juju 2.8.10 we are setting the pod spec on every upgrade-charm hook (something I don-t like either because we get pod rollback on every upgrade of the charms), and its failing because there is no pod spec associated to the operator charms.

[Ian Booth (wallyworld)]

Correct, pod-spec-set is not (currently) supported on operator charms. Juju does not attempt to track any workload pods for these charms. Does you charm have a pod spec that it wants to use? What resources are in that pod spec?

[David (davigar15)]

Ian: this is the example charm that is not setting any pod spec. As we thought, this could be an Operator Framework bug, and not juju.

https://github.com/davigar15/kdu-proxy-operator

[Ian Booth (wallyworld)]

It was an issue in juju - we always do a pod spec set on upgrade to increment the charm modified version. We just need to allow spec = nil to go through. PR will also remove the feature flag.

https://github.com/juju/juju/pull/12984