Try it out: if the source text file has an empty string (per the original post), the Retain NULLs option will treat that empty string as a NULL and not add/convert to that dummy date. If the string field in question is white space, and not actually an empty string, then yes, the field would need to be NULLed in a derived column task. I ran into this recently myself (with the odd dates in a field that I know was empty in the source file), and this bit fixed it.

This bit has nothing to do with the destination, which is what you're referring to - as long as the destination allows NULLs, which the OP said it does, enabling the retention of NULLs at the source will pass a NULL to the destination ... a derived column task would do the trick, too, but it is extra work/unnecessary with this bit.