Fix RPC get_worker_info for rank=0 (#52804)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52804
`rpc.get_worker_info` used to only take string in v1.6. We recently
allow it to accept `int` and `WorkerInfo`, but the previous check
on `worker_name` is no longer correct. This commit adds explicit
`not None` check.
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D26655089
Pulled By: mrshenli
fbshipit-source-id: fa1545bd6dd2b33bc1e919de46b94e799ab9719c
