juju status prints wrong os for sidecar charms without workload containers

[Leon]

In the following `juju status` ouput there are two charms that do not have a workload container specified in metadata.yaml.
As a results, `juju status` prints "ubuntu" instead of "kubernetes" in the OS column.

App Version Status Scale Charm Store Channel Rev OS Address Message
alertmanager active 1 alertmanager-k8s charmhub edge 9 kubernetes 10.152.183.87
cos-config active 1 cos-configuration-k8s charmhub edge 5 kubernetes 10.152.183.247
grafana active 1 grafana-k8s charmhub edge 23 kubernetes 10.152.183.205
loki active 1 loki-k8s charmhub edge 15 kubernetes 10.152.183.167
prometheus active 1 prometheus-k8s charmhub edge 19 kubernetes 10.152.183.34
scrape-config active 1 prometheus-scrape-config-k8s charmhub edge 21 ubuntu 10.152.183.7
scrape-target active 1 prometheus-scrape-target-k8s charmhub edge 9 ubuntu 10.152.183.57

[Leon (sed-i)]

~~rbarry kindly pointed out that the case statement in

https://github.com/juju/juju/blob/efcf4c6acfb41c6eeb01f9f03cb96c4b3a507a04/core/charm/kubernetes.go#L16

falls through because there are no containers defined

[John A Meinel (jameinel)]

We'll look to tackle this as part of fixing our handling of 'series' vs 'os/version'.