The documentation suggests that what you want is not possible:
To make it as easy as possible to interface with modules which implement the DB API, the value None is written as the empty string.
This is in the documentation for the
writer class, suggesting it is true for all dialects and is an intrinsic limitation of the csv module.
I for one would support changing this (along with various other limitations of the csv module), but it may be that people would want to offload this sort of work into a different library, and keep the CSV module simple (or at least as simple as it is).
If you need more powerful file-reading capabilities, you might want to look at the CSV reading functions in numpy, scipy, and pandas, which as I recall have more options.