Postcss exits process when file source array is empty.

[vilmosioo]

This breaks any watch task that uses grunt-postcss.

[jayenashar]

i'm having a different issue that may be related to this one.

https://github.com/nDmitry/grunt-postcss/blob/master/tasks/postcss.js#L124 seems to return while in a loop. not only could this cause @vilmosioo's issue, but it also seems to cause havoc with other async tasks that run after. (i'm having trouble where grunt doesn't wait for svgmin to finish before it moves on to the next task/exiting.)