Fix panic if Metadata is nil

[jokemanfire]

Metadata is nil will cause panic

/kind bug

When use the cri interface to contact with my server, I miss set the Metadata(its nil),and it will cause a panic.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xfc64f1]

goroutine 1 [running]:
main.getSandboxesList({0xc0003e0040, 0x1, 0x126c9da?}, 0xc00051f818)
/home/runner/work/cri-tools/cri-tools/cmd/crictl/sandbox.go:693 +0x91
main.ListPodSandboxes({0x1452120, 0xc000455920}, 0xc00051f818)
/home/runner/work/cri-tools/cri-tools/cmd/crictl/sandbox.go:570 +0x36d
main.OutputPodSandboxes({0x1452120?, 0xc000455920?}, 0xc00051f818)
/home/runner/work/cri-tools/cri-tools/cmd/crictl/sandbox.go:576 +0x3a
main.init.func34(0xc0004a0cc0)
/home/runner/work/cri-tools/cri-tools/cmd/crictl/sandbox.go:377 +0x2ae
github.com/urfave/cli/v2.(*Command).Run(0x1d72fa0, 0xc0004a0cc0, {0xc000238980, 0x1, 0x1})
/home/runner/work/cri-tools/cri-tools/vendor/github.com/urfave/cli/v2/command.go:276 +0x97d
github.com/urfave/cli/v2.(*Command).Run(0xc0003d22c0, 0xc0004a07c0, {0xc00012c120, 0x6, 0x6})
/home/runner/work/cri-tools/cri-tools/vendor/github.com/urfave/cli/v2/command.go:269 +0xbb7
github.com/urfave/cli/v2.(*App).RunContext(0xc0004a6000, {0x1440ae0, 0x1e03520}, {0xc00012c120, 0x6, 0x6})
/home/runner/work/cri-tools/cri-tools/vendor/github.com/urfave/cli/v2/app.go:333 +0x5a5
github.com/urfave/cli/v2.(*App).Run(...)
/home/runner/work/cri-tools/cri-tools/vendor/github.com/urfave/cli/v2/app.go:307
main.main()
/home/runner/work/cri-tools/cri-tools/cmd/crictl/main.go:363 +0xcf2

[linux-foundation-easycla]

• ✅login: jokemanfire / (94a6e23)

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jokemanfire, saschagrunert

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