Symptoms:

You have a report that runs perfectly fine and can confirm it has content.
You shedule it to run but when it arrives you have a zero kb zip file.

Solution:

Came across this today because the report name had an apostrohe in it. ex; John's Report

Cause:

When scheduling a report the name is passed as a paramater in a sql query. The ' charachter causes an end of field and terminates the report before it can run.

* I havent confirmed but suspect other charachters such as % would cause issue as well