bug(shutdown): Fix panic when webhooks are disabled

[zeeke]

When webooks are disabled, shutdown procedure produces the following panic error:

2024-08-13T12:45:04.971685297Z	INFO	shutdown	utils/shutdown.go:22	Done clearing finalizers on exit
2024-08-13T12:45:04.971713179Z	INFO	shutdown	utils/shutdown.go:23	Seting webhook failure policies to Ignore on exit
2024-08-13T12:45:04.978386488Z	ERROR	shutdown	utils/shutdown.go:64	Error getting webhook	{"error": "validatingwebhookconfigurations.admissionregistration.k8s.io \"sriov-operator-webhook-config\" not found"}
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils.updateValidatingWebhook(0x37d7788?)
	/go/src/github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils/shutdown.go:75 +0x198
github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils.updateWebhooks()
	/go/src/github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils/shutdown.go:64 +0xa5
github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils.Shutdown()
	/go/src/github.com/k8snetworkplumbingwg/sriov-network-operator/pkg/utils/shutdown.go:23 +0x14
main.main()
	/go/src/github.com/k8snetworkplumbingwg/sriov-network-operator/main.go:296 +0x1e6a

Fix the panic error and add an end2end test case to cover it.

[github-actions]

Thanks for your PR,
To run vendors CIs, Maintainers can use one of:

• /test-all: To run all tests for all vendors.
• /test-e2e-all: To run all E2E tests for all vendors.
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To skip the vendors CIs, Maintainers can use one of:

• /skip-all: To skip all tests for all vendors.
• /skip-e2e-all: To skip all E2E tests for all vendors.
• /skip-e2e-nvidia-all: To skip all E2E tests for NVIDIA vendor.
  Best regards.

[coveralls]

Pull Request Test Coverage Report for Build 10487195141

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 44.976%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/utils/shutdown.go	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
controllers/drain_controller.go	3	67.1%

Totals
Change from base Build 10454628601:	-0.02%
Covered Lines:	6530
Relevant Lines:	14519

---

💛 - Coveralls