The new logic for optimizing outdated file checks made an assumption

that the destination file always exists. It tried comparing the source

file's timestamp to the destination's, which would be None. This would

execute just fine on Python 2.7, but not on Python 3, where these types

are not comparable.
This change fixes this by short-circuiting the timestamp comparisons if

None. Unit tests were added to ensure this works correctly on all

versions of Python.

Unit tests pass for all versions of Python.
Encountered this issue during development of a new .less file, and

verified this fixed it.