The doc string of the variable 'csv-comment-start' states that it is allowed to
be nil:

   String that starts a comment line, or nil if no comment syntax.

However, since commit 388bcb9a77118d7b7d9c42aa8b3f9fa2c892b930 (Fix
'csv-guess-separator' being confused by comments) this is no longer the case,
because the variable is passed to 'string-to-char', which does not accept nil as
input.

As a result, if 'csv-comment-start' is nil, the separator cannot be guessed
anymore, which may result in the wrong separator being guessed.

Recipe to reproduce:

- Start emacs -Q -L /path/to/csv-mode-1.27 (the current version on ELPA).
- Load 'csv-mode.el'.
- Set 'csv-comment-start-default' to nil.
- Open a CSV file with non-standard separator (e.g., TAB, though make sure the
  file extension is '.csv', not '.tsv'.)
- Check the buffer-local value of 'csv-separators': it should be ("\t"), but it
  is ("," "\t").

If you try to add a column to a line (hit TAB with point at EOL), you'll notice
a comma being added as separator.

Tiny patch attached.