puni-kill-active-region: do Emacs default behavior if there's no active region

[tsdh]

In Emacs 31, there's a new user option.

kill-region-dwim is a variable defined in ‘simple.el’.

Its value is ‘emacs-word’
Original value was nil

Behavior when ‘kill-region’ is invoked without an active region.
If set to nil (default), kill the region even if it is inactive,
signaling an error if there is no region.
If set to ‘emacs-word’, kill the last word as defined by the
current major mode.
If set to ‘unix-word’, kill the last word in the style of a shell like
Bash.  This ignores the major mode like ‘unix-word-rubout’ (which see).

  This variable was introduced, or its default value was changed, in
  version 31.1 of Emacs.
  You can customize this variable.

So when set to emacs-word (or unix-word) and you hit C-w (kill-region) while there is no active region, it will just kill the previous word. That's a very nice feature. However, it won't work with puni-kill-active-region which always signals an error when there's no active region. This PR changes that so that erroring itself, we simply fall back to kill-region.