When requesting file data, make "Range: bytes=0-" work correctly

Summary: Ref T12219. Chrome can send requests with a "Range: bytes=0-" header, which just means "the whole file", but we don't respond correctly because of a null vs 0 issue.

Test Plan: Sent a raw bytes=0- request, saw a proper resonse.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12219

Differential Revision: https://secure.phabricator.com/D17627