fix(cli): exit when calling subcommand node without args

[book987]

Checklist:

• My organization is added to USERS.md.

argo node without args didn't exit after displaying help docs, which cause an index out of range panic in next line.

pls take a look @alexec

λ argo node
perform action on a node in a workflow

Usage:
  argo node ACTION WORKFLOW FLAGS [flags]

Examples:
# Set outputs to a node within a workflow:

  argo node set my-wf --output-parameter parameter-name="Hello, world!" --node-field-selector displayName=approve

# Set the message of a node within a workflow:

  argo node set my-wf --message "We did it!"" --node-field-selector displayName=approve


Flags:
  -h, --help                           help for node
  -m, --message string                 Set the message of a node, eg: --message "Hello, world!"
      --node-field-selector string     Selector of node to set, eg: --node-field-selector inputs.paramaters.myparam.value=abc
  -p, --output-parameter stringArray   Set a "supplied" output parameter of node, eg: --output-parameter parameter-name="Hello, world!"
      --phase string                   Phase to set the node to, eg: --phase Succeeded

Global Flags:
      --argo-base-href string          An path to use with HTTP client (e.g. due to BASE_HREF). Defaults to the ARGO_BASE_HREF environment variable.
      --argo-http1                     If true, use the HTTP client. Defaults to the ARGO_HTTP1 environment variable.
  -s, --argo-server host:port          API server host:port. e.g. localhost:2746. Defaults to the ARGO_SERVER environment variable.
      --as string                      Username to impersonate for the operation
      --as-group stringArray           Group to impersonate for the operation, this flag can be repeated to specify multiple groups.
      --certificate-authority string   Path to a cert file for the certificate authority
      --client-certificate string      Path to a client certificate file for TLS
      --client-key string              Path to a client key file for TLS
      --cluster string                 The name of the kubeconfig cluster to use
      --context string                 The name of the kubeconfig context to use
      --gloglevel int                  Set the glog logging level
      --insecure-skip-tls-verify       If true, the server's certificate will not be checked for validity. This will make your HTTPS connections insecure
  -k, --insecure-skip-verify           If true, the Argo Server's certificate will not be checked for validity. This will make your HTTPS connections insecure. Defaults to the ARGO_INSECURE_SKIP_VERIFY environment variable.
      --instanceid string              submit with a specific controller's instance id label. Default to the ARGO_INSTANCEID environment variable.
      --kubeconfig string              Path to a kube config. Only required if out-of-cluster
      --loglevel string                Set the logging level. One of: debug|info|warn|error (default "info")
  -n, --namespace string               If present, the namespace scope for this CLI request
      --password string                Password for basic authentication to the API server
      --request-timeout string         The length of time to wait before giving up on a single server request. Non-zero values should contain a corresponding time unit (e.g. 1s, 2m, 3h). A value of zero means don't timeout requests. (default "0")
  -e, --secure                         Whether or not the server is using TLS with the Argo Server. Defaults to the ARGO_SECURE environment variable.
      --server string                  The address and port of the Kubernetes API server
      --token string                   Bearer token for authentication to the API server
      --user string                    The name of the kubeconfig user to use
      --username string                Username for basic authentication to the API server
  -v, --verbose                        Enabled verbose logging, i.e. --loglevel debug
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/argoproj/argo/v2/cmd/argo/commands.NewNodeCommand.func1(0xc0001e8000, 0x4d8ac80, 0x0, 0x0)
	/Users/sbeharyutk/go/src/github.com/argoproj/argo/cmd/argo/commands/node.go:47 +0x8ee
github.com/spf13/cobra.(*Command).execute(0xc0001e8000, 0x4d8ac80, 0x0, 0x0, 0xc0001e8000, 0x4d8ac80)
	/Users/sbeharyutk/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:846 +0x2aa
github.com/spf13/cobra.(*Command).ExecuteC(0xc000154580, 0xc00007c750, 0xc00087ff50, 0x100545f)
	/Users/sbeharyutk/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950 +0x349
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/sbeharyutk/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887
main.main()
	/Users/sbeharyutk/go/src/github.com/argoproj/argo/cmd/argo/main.go:13 +0x2b

[codecov]

Codecov Report

Merging #5556 (17ff222) into master (20f0047) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

❗ Current head 17ff222 differs from pull request most recent head d00cc4e. Consider uploading reports for the commit d00cc4e to get more accurate results

@@            Coverage Diff             @@
##           master    #5556      +/-   ##
==========================================
- Coverage   47.04%   47.00%   -0.05%     
==========================================
  Files         240      240              
  Lines       15012    15013       +1     
==========================================
- Hits         7063     7057       -6     
- Misses       7051     7057       +6     
- Partials      898      899       +1     

Impacted Files	Coverage Δ
cmd/argo/commands/node.go	0.00% <0.00%> (ø)
cmd/argoexec/commands/emissary.go	48.43% <0.00%> (-1.57%)	⬇️
cmd/argo/commands/get.go	56.00% <0.00%> (-1.34%)	⬇️

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[simster7]

Thanks!