bug(vault): Fix panic after token expires

[louisruch]

Created a Vault store, expired it in Vault and waited for Boundary to expire to token. I then ran terraform apply which lead to the panic:

anic: interface conversion: interface {} is nil, not string
2022-09-27T16:48:09.875-0700 [DEBUG] plugin.terraform-provider-boundary:
2022-09-27T16:48:09.875-0700 [DEBUG] plugin.terraform-provider-boundary: goroutine 140 [running]:
2022-09-27T16:48:09.875-0700 [DEBUG] plugin.terraform-provider-boundary: github.com/hashicorp/terraform-provider-boundary/internal/provider.setFromVaultCredentialStoreResponseMap(0xc000209310?, 0x4fc9188?, 0x1)
2022-09-27T16:48:09.875-0700 [DEBUG] plugin.terraform-provider-boundary: 	/Users/louisruch/terraform-provider-boundary/internal/provider/resource_credential_store_vault.go:146 +0x525

Rebuilt with the new code changes and it did not panic but gave the following warning:

[louisruch]

@johanbrandhorst @ddebko This fixes a panic that was occurring because the Vault Token HMAC is not returned for expired tokens, only the token_status is returned. I added details of my test to the description

[johanbrandhorst]

Nice work!