rabbitmq_permissions: Fix error on empty permissions.

[cyrilgdn]

We want to be able to create permissions for a user with no permissions.

e.g:

resource "rabbitmq_permissions" "readonly" {
  user  = "test"
  vhost = "/"

  permissions {
    configure = ""
    write     = ""
    read      = ""
  }
}

When putting only empty string, Terraform puts nil in the list of permissions object
instead of a map with 3 empty strings.

fix #42

[aminueza]

I've tested the changes and it's working perfectly. You can merge this PR. Thank you @cyrilgdn.

[cyrilgdn]

@aminueza Thanks a lot for your time on this review 👍