Modify function whether region is active or not

Use region-active-p function to prepare region or not instead of use-region-p.
Because use-region-p couldn't expand region if use-empty-active-region
is nil.
If someone wants to use use-region-p, please override mykie:region-p by advice.

Code sample:
(defun my/region-extender (&optional input)
  (interactive)
  (set-mark
   (save-excursion
     (goto-char (mark))
     (call-interactively
      (pcase input
        (`"n"  'next-line)
        (`"p"  'previous-line)
        (`"b"  'backward-char)
        (input 'forward-char)))
     (point))))