NoMethodError when await_workflow_result is called for long running workflows

Seeing the below error when calling await_workflow_result for longer workflows.
```
await_workflow_result': undefined method 'type' for nil:NilClass (NoMethodError)
```
The current implementation assumes that the result will always be returned with non-empty events list if a request doesn't time out from GRPC::DeadlineExceeded. This is not the case when a request is created with the wait_new_event option set to true. Temporal sever treats this as a long poll request and populates the next_page_token to inform that client that it needs to keep polling. This process is repeated until an empty next_page_token is sent alongside the new event.