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By default if you don't have a robots.txt it assumes everything on your site is freely crawable, if it is you don't have the need for a robots file. If however you don't want it crawling a certain page you can indicate this in the file.
This page has more details (hope it is ok to list, sure i have seen it here before)
But to answer your quesion, the lack of a robots.txt file won't be hurting your rankings.