fix: Print valid JSON/YAML when workflow list empty #10873

[Goorzhel]

Fixes #10873.

Motivation

Tools with -o json are expected to output valid JSON. Same for YAML.

Modifications

I kept the change minimal. Since PrintWorkflows fails fast at len(workflows) == 0, I stuck with that.

Verification

$ make dist/argo-linux-amd64
<...>
$ dist/argo-linux-amd64 -v list -o json
DEBU[2023-08-25T23:26:48.261Z] CLI version                                   version="{latest+9cc740c.dirty 2023-08-26T05:59:15Z 9cc740cc49a9ff91a955bd1356dbacb8dcf4fe50 untagged dirty go1.20.4 gc linux/amd64}"
DEBU[2023-08-25T23:26:48.261Z] Client options                                opts="(argoServerOpts=(url=,path=,secure=true,insecureSkipVerify=false,http=false),instanceID=)"
I0825 23:26:48.263036 1745777 loader.go:372] Config loaded from file:  /home/ag/.kube/config
I0825 23:26:48.264209 1745777 loader.go:372] Config loaded from file:  /home/ag/.kube/config
DEBU[2023-08-25T23:26:48.264Z]                                               listOpts="&ListOptions{LabelSelector:,FieldSelector:,Watch:false,ResourceVersion:,TimeoutSeconds:nil,Limit:0,Continue:,AllowWatchBookmarks:false,ResourceVersionMatch:,}"
I0825 23:26:48.270409 1745777 round_trippers.go:553] GET https://<KUBE_API>:6443/apis/argoproj.io/v1alpha1/namespaces/default/workflows?labelSelector=%21workflows.argoproj.io%2Fcontroller-instanceid 200 OK in 5 milliseconds
DEBU[2023-08-25T23:26:48.273Z] CanI                                          name= namespace=default resource=workflows verb=list
I0825 23:26:48.275370 1745777 round_trippers.go:553] POST https://<KUBE_API>:6443/apis/authorization.k8s.io/v1/selfsubjectaccessreviews 201 Created in 1 milliseconds
DEBU[2023-08-25T23:26:48.275Z] CanI                                          name= namespace=default resource=workflows status="{true false  }" verb=list
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