Backport of Fix panic when unlocking unlocked user into release/1.15.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #23611 to be assessed for backporting due to the inclusion of the label backport/1.15.x.

The below text is copied from the body of the original PR.

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This PR fixes a panic when attempting to unlock an already unlocked user.

Example Panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x380c80a]

goroutine 147274 [running]:
github.com/hashicorp/vault/vault.unlockUser({0xa434c80, 0xc00fa6aeb0}, 0xc00ef26900, {0xc00e8cfb11, 0x16}, {0xc00e8cfb2f, 0x6})
	/home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/vault/logical_system_user_lockout.go:61 +0x28a
github.com/hashicorp/vault/vault.(*SystemBackend).handleUnlockUser(0xc005b7e740, {0xa434c80, 0xc00fa6aeb0}, 0x0?, 0xffffffffffffffff?)
	/home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/vault/logical_system.go:2492 +0x27b
github.com/hashicorp/vault/sdk/framework.(*Backend).HandleRequest(0xc011fbd770, {0xa434c80, 0xc00fa6aeb0}, 0xc023034fc0)
	/home/runner/actions-runner/_work/vault-enterprise/vault-enterprise/sdk/framework/backend.go:310 +0xa88

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Overview of commits

• 2f8e59c

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