Both the transform and reverseTransform methods perform the same action with regards to null / empty strings. Both functions check if the value is null and return an empty string. Judging by the DocBlock comments in the DataTransformerInterface the behaviour of reverseTransform should be to check if the passed value is an empty string, and return NULL in it's place, thus performing the inverse action of the transform method.

Edit: Thought I'd best mention my version...
"package": "symfony/symfony",
"version": "dev-master",
"source-reference": "d0e154725c85a3ec5bc6760a59cc2769b8a17ae5"