Make lint tests handle paths better and distinguish between "0" and "null" more carefully

Summary:
Ref T13543. Currently, the cpplint tests do not function because cpplint is passed a path which does not end in a suffix it recognizes.

Change the tempfile / path code to pass linter path/to/example.c-style linters a path they expect.

Then, correct some older code which was playing it fast-and-loose with "null" vs "0".

Test Plan: Ran arc unit --everything, got a clean bill of health on all the linters I have installed. (This is probably not all tests, since I have only a subset of linters installed locally that we have code for.)

Maniphest Tasks: T13543

Differential Revision: https://secure.phabricator.com/D21291