Fix qsprintf() '%nd' conversion

Summary:
I broke this a little bit in my overzealous D1174, since this block validates
both '%nd' (nullable integer) and '%d' (non-nullable integer).

Clean up the conditional checks so we catch the bad case ('%d' on a PHID
converting to 0) but let the good case ('%nd' with null) through.

Test Plan: Unit tests failed; applied patch; unit tests pass.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T670

Differential Revision: 1201