Fix "empty diff" error in arcanist

Summary: Ref T2296. This error is unreachable right now -- when I fixed all the "\r\n" stuff, we always end up with a nonempty first line for an empty input. Do this test earlier and more explicitly. This results in a less useful error: "expected (some junk) on line 1" instead of "can't parse an empty diff".

Test Plan: Tried to parse an empty diff, got a "you can't parse an empty diff" error.

Reviewers: btrahan, vrana, codeblock

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2296

Differential Revision: https://secure.phabricator.com/D4370