There are cases where a diffx diff section will have empty diff

content--for example, when a file is renamed but doesn't have any

changes to content. The diff stats generation will do the right thing if

a file is added without specifying diff= in the constructor at all,

but was spewing errors with diff=b''. This change updates things to

check for falsiness rather than explicitly checking against None.

Ran with some code that was creating empty (but not None) diffs. Saw no

  more errors.
Ran unit tests.