Variable: comment-empty-lines
comment-empty-lines is a customizable variable defined in
newcomment.el.gz.
Value
nil
Documentation
If nil, comment-region does not comment out empty lines.
If t, it always comments out empty lines.
If eol, it only comments out empty lines if comments are
terminated by the end of line (i.e., comment-end is empty).
Source Code
;; Defined in /usr/src/emacs/lisp/newcomment.el.gz
(defcustom comment-empty-lines nil
"If nil, `comment-region' does not comment out empty lines.
If t, it always comments out empty lines.
If `eol', it only comments out empty lines if comments are
terminated by the end of line (i.e., `comment-end' is empty)."
:type '(choice (const :tag "Never" nil)
(const :tag "Always" t)
(const :tag "EOL-terminated" eol))
:group 'comment)