[BUG] org-capture-kill: doesn't remove the right text region

[Caleb Chase <caleb@chasecaleb.com>]

I discovered a bug with org-capture and org-capture kill when
canceling an item or plain type template on a heading that has only an
empty line for content. Minimal reproduction is as follows:

1. Add the following to =/tmp/capture-bug-init.el=:
   #+begin_src emacs-lisp :noeval t
     (require 'org)
     (setq org-capture-templates
           '(("i" "Repro: item type" item (file+headline
"/tmp/test-capture.org" "Foo") nil :unnarrowed t)
             ("p" "Repro: plain type" plain (file+headline
"/tmp/test-capture.org" "Foo") "Plain capture: %?" :unnarrowed t)))

     (with-temp-buffer
       ;; FIXME BUG: occurs with empty line between headings.
       (insert "* Foo\n\n* Bar\n")
       ;; FIXME WORKING: no empty line between headings.
       ;; (insert "* Foo\n* Bar\n")
       ;; FIXME WORKING: existing item between headings.
       ;; (insert "* Foo\n- Some note\n* Bar\n")
       (write-file "/tmp/test-capture.org"))
     (find-file "/tmp/test-capture.org")
     (org-capture nil "i")
   #+end_src
2. Run Emacs: =emacs -Q --load /tmp/capture-bug-init.el=
3. Cancel the capture with =C-c C-k=
4. Bug occurs: the dash is left behind, so the last line is now =-*
Bar= instead of =* Bar=.

You can also reproduce this bug with the plain type template I have
above. Notice the different commented-out variants illustrating when
it behaves properly. This bug occurs when there is a single blank
empty line between the two headings, but does not occur if there is
not empty line or if there is an existing item.

---
*POSSIBLE ROOT CAUSE*
I suspect the =:begin-marker= capture marker is determined incorrectly
for plain and item capture types. I haven't dug into the relevant
functions, =org-capture-place-item= and =org-capture-place-plain-text=
closely enough to know exactly what is going on, but here's a
workaround I came up with. It's obviously not the Right Solution (TM),
but at least it illustrates the problem:

#+begin_src emacs-lisp :noeval t
  (defun capture-cancel-bug-fix ()
    (let ((begin (org-capture-get :begin-marker 'local)))
      (unless (equal (char-before begin) (string-to-char "\n"))
        (setq org-capture-current-plist
              (plist-put org-capture-current-plist
                         :begin-marker (1- begin))))))
  (add-hook 'org-capture-mode-hook #'capture-cancel-bug-fix)
#+end_src

---
*CONTEXT*
- Emacs version: 28.1
- Org version: 9.5.4. More specifically, tested on current bugfix
branch revision 6dc785288d3514af4071f210dac0a18c14a6c45b and current
main branch revision d9479887226ad79a1a8de739e7be0fc1fffec536.