SourceUrl option not working for modules with defineInfos.length === 0

Hi James,

I'm using the sourceUrl option on my dev environment and realized it was not working for some files (named modules for example but not only).

Here's the reason why : 
The sourceUrl magic happens at the very end of the transform.toTransport function. But higher in the code, if ever the module has a defineInfos.length === 0, the "contents" variable is returned without being wrapped in the eval statement.

My pull request fixes that issue.