vault: fix panic by checking for nil secret
vault: fix panic by checking for nil secret

Vault's RenewSelf(...) API may return (nil, nil). We failed to check if
secret was nil before attempting to use it.

RenewSelf:
https://github.com/hashicorp/vault/blob/e3eee5b4fb386418d10b6a248252ec6e2e05d980/api/auth_token.go#L138-L155

Calls ParseSecret:
https://github.com/hashicorp/vault/blob/e3eee5b4fb386418d10b6a248252ec6e2e05d980/api/secret.go#L309-L311

If anyone has an idea on how to test this I didn't see any options. We
use a real Vault service, so there's no opportunity to mock the
response.

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