do not add request logs if thread does not have request context
do not add request logs if thread does not have request context

Summary:
avoid taking the memory hit of adding it to the fallback RCTX returned by
`folly::RequestContext::get`

Reviewed By: praihan

Differential Revision: D32050369

fbshipit-source-id: fbda373557acde56d23ead22c06fafaee227f096

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