Fix a panic I observed in a sealed node running CollectMetrics

[ncabatoff]

... where RaftBackend.raft has been set to nil by TeardownCluster:

TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: 2023-06-15T10:12:29.670-0400 [INFO]  core: vault is sealed
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: panic: runtime error: invalid memory address or nil pointer dereference
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x27e2631]
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: 
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: goroutine 137 [running]:
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: github.com/hashicorp/raft.(*Raft).Stats(0x0)
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4:         /Users/runner/go/pkg/mod/github.com/hashicorp/[email protected]/api.go:1132 +0x31
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: github.com/hashicorp/vault/physical/raft.(*RaftBackend).CollectMetrics(0xc0006a06e0, 0xc001a46400?)
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4:         /Users/runner/actions-runner/_work/vault-enterprise/vault-enterprise/physical/raft/raft.go:630 +0x125
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: github.com/hashicorp/vault/vault.(*Core).metricsLoop(0xc000627200, 0xc001110600)
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4:         /Users/runner/actions-runner/_work/vault-enterprise/vault-enterprise/vault/core_metrics.go:121 +0xe8b
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: github.com/hashicorp/vault/vault.(*Core).runStandby.func7()
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4:         /Users/runner/actions-runner/_work/vault-enterprise/vault-enterprise/vault/ha.go:446 +0x25
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: github.com/oklog/run.(*Group).Run.func1({0xc0016beac8?, 0xc0016beae0?})
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4:         /Users/runner/go/pkg/mod/github.com/oklog/[email protected]/group.go:38 +0x2f
TestVaultExecClusterMigrateShamirToTransit-vault-srv-4: created by github.com/oklog/run.(*Group).Run

[banks]

LGTM