client-gen: avoid panic for empty groups

[sttts]

No description provided.

[mrIncompetent]

I ran into this issue when i tried to generate a client for 2 groups.
This PR fixes my problem!

Removing old client
Generating clientset
panic: runtime error: index out of range

goroutine 1 [running]:
github.com/kubermatic/kubermatic/api/vendor/k8s.io/code-generator/cmd/client-gen/generators.(*genGroup).Filter(0xc4286b5b80, 0xc428f1b980, 0xc425f67290, 0x882688)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/code-generator/cmd/client-gen/generators/generator_for_group.go:49 +0x3e
github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator.(Generator).Filter-fm(0xc428f1b980, 0xc425f67290, 0xc42bbd9828)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator/execute.go:221 +0x43
github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator.(*Context).filteredBy(0xc428f1b980, 0xc42bbd9ab8, 0x882688)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator/execute.go:182 +0x108
github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator.(*Context).ExecutePackage(0xc428654600, 0xc42000c9a0, 0x15, 0x84d5a0, 0xc42a69a200, 0x0, 0x0)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator/execute.go:221 +0x3c6
github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator.(*Context).ExecutePackages(0xc428654600, 0xc42000c9a0, 0x15, 0xc42a69aa00, 0x7, 0x8, 0x0, 0xc420017140)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/generator/execute.go:51 +0xa7
github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/args.(*GeneratorArgs).Execute(0xc420078500, 0xc420017290, 0x70029d, 0x6, 0x7147d8, 0x1, 0x4)
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/gengo/args/args.go:166 +0x22c
main.main()
	/home/henrik/work/src/github.com/kubermatic/kubermatic/api/vendor/k8s.io/code-generator/cmd/client-gen/main.go:201 +0x650

[wojtek-t]

/lgtm
/approve no-issue

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sttts, wojtek-t

Associated issue requirement bypassed by: wojtek-t

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these OWNERS Files:

• staging/src/k8s.io/code-generator/cmd/client-gen/OWNERS [wojtek-t]

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[sttts]

/retest

[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-ci-robot]

@sttts: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
pull-kubernetes-e2e-kops-aws	4039a72	link	/test pull-kubernetes-e2e-kops-aws

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 52073, 52079)

[lavalamp]

@grodrigues3 This PR is fine and good, but the queue shouldn't have merged it. Did someone forget to make the queue require the approved for milestone label??

[lavalamp]

@caesarxuchao can you double check this PR?

[caesarxuchao]

lgtm