Change Csv comment character to None to fix regression from #2812

As @amras1 noted in #2716:

"Looks like the rebase on #2812 introduced a test error resulting from the fact that legacy CSV functionality has now changed so that '#' is the default comment character (previously there had been no default comment character for CSV). Is this an unintended consequence of the restructuring in #2812?"

This PR reverts the comment characters for the `Csv` reader back to `None`.

@hamogu @amras1 - does this look reasonable?  No CHANGES.rst update since this regression never got released.