kubernetes auth role: allow unset audience

[davidmontoyago]

Community Note

• Please vote on this pull request by adding a 👍 reaction to the original pull request comment to help the community and maintainers prioritize this request
• Please do not leave "+1" comments, they generate extra noise for pull request followers and do not help prioritize the request

Closes #868

Release note for CHANGELOG:

resource/vault_kubernetes_auth_backend_role: allow unset audience.

Output from acceptance testing:

TF_ACC=1 go test -v github.com/hashicorp/terraform-provider-vault/vault -run TestAccKubernetesAuthBackendRole_fullUpdate

--- PASS: TestAccKubernetesAuthBackendRole_fullUpdate (43.02s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     43.581s
...

[tvoran]

Works well, thanks!