kubectl-mtb segfaults if a given benchmark doesn't exist

[Mac Chaffee]

$ kubectl-mtb get benchmark foo
ID    NAME   CATEGORY   TYPE   PROFILE LEVEL
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1cb7653]

goroutine 1 [running]:
sigs.k8s.io/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb.printBenchmarks()
	/Users/machaffe/personal/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb/get.go:64 +0x213
sigs.k8s.io/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb.glob..func2(0x29f6b40, {0xc00070f180, 0x2, 0x2})
	/Users/machaffe/personal/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb/get.go:43 +0x19
github.com/spf13/cobra.(*Command).execute(0x29f6b40, {0xc00070ee60, 0x2, 0x2})
	/Users/machaffe/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:846 +0x5f8
github.com/spf13/cobra.(*Command).ExecuteC(0xc0002cf080)
	/Users/machaffe/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950 +0x3ad
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/machaffe/go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887
sigs.k8s.io/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb.Execute()
	/Users/machaffe/personal/multi-tenancy/benchmarks/kubectl-mtb/internal/kubectl-mtb/root.go:51 +0x25
main.main()
	/Users/machaffe/personal/multi-tenancy/benchmarks/kubectl-mtb/cmd/kubectl-mtb/main.go:18 +0x17

I think it may be caused by filterBenchmarks(). Working on a fix now