Without current_tenant, tenant field shuld be allowed to be nil.

If current_tenant is not set, I need to create a new document without specifying a value to the tenant field.

Some background.
Our application/service start without multi-tenancy, personal accounts. Then we are going to introduce more strict private `team` as multi-tenancy. We need to live those personal documents with team multi-tenancy in one application and mongo database.

`optional: true` option is not a resolution, because;
- The option does not change `validates presence` behavior, only not to set tenant field before_validation.
- In my case documents without a tenant field are not "shared" document. These documents must be excluded in queries if current_tenant is set.

I think if `current_tenant` is not set, the application must behave in the world without multi-tenancy.