Colorspace now allows empty file representation list

[jrsndl]

Changelog Description

Colorspace.py now check for empty file representation list.

Testing notes:

1. In Nuke, make sure render work folder (staging dir) has no rendered frames
2. Try to publish on farm with existing frames
3. Publisher should stop on rendered frames validatior, not on colorspace error

Can fix #97 Might also fix #66