fix: panic when environment variables are empty

[NgZiming]

On mac OS, environment variables like: OLDPWD are usually empty. May trigger panics like:

😄  minikube v1.26.0 on Darwin 11.6.5
panic: runtime error: index out of range [1] with length 1

goroutine 1 [running]:
k8s.io/minikube/cmd/minikube/cmd.displayEnviron({0xc000a69a40?, 0x1c, 0xb?})
	/private/tmp/minikube-20220623-65769-c2xa25/cmd/minikube/cmd/start.go:445 +0x252
k8s.io/minikube/cmd/minikube/cmd.runStart(0x7316040?, {0x5d07e13?, 0x0?, 0x0?})
	/private/tmp/minikube-20220623-65769-c2xa25/cmd/minikube/cmd/start.go:157 +0x245
github.com/spf13/cobra.(*Command).execute(0x7316040, {0x73736d8, 0x0, 0x0})
	/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:876 +0x67b
github.com/spf13/cobra.(*Command).ExecuteC(0x7313fc0)
	/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:990 +0x3b4
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/brew/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.5.0/command.go:918
k8s.io/minikube/cmd/minikube/cmd.Execute()
	/private/tmp/minikube-20220623-65769-c2xa25/cmd/minikube/cmd/root.go:170 +0xca5
main.main()
	/private/tmp/minikube-20220623-65769-c2xa25/cmd/minikube/main.go:88 +0x255

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• ✅ login: NgZiming / name: 吴梓铭 (360cc86)

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @NgZiming. Thanks for your PR.

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[minikube-bot]

Can one of the admins verify this patch?

[spowelljr]

Thanks for the fix!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: NgZiming, spowelljr

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[medyagh]

nice catch, thank you @NgZiming