Special-case /dev/null in open in the portability header
Special-case /dev/null in open in the portability header

Summary: `/dev/null` doesn't exist on Windows, but thankfully, `NUL` does, and has the same semantics.

Reviewed By: meyering

Differential Revision: D3698007

fbshipit-source-id: 5ef31c6576f988dd747ea3c39e296c244bc640b7

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