Work for empty MIME attachments (related to bug#34387)

* mm-view.el (mm-inline-text):
Make undisplayer do nothing if the part is empty.

* mm-decode.el (mm-copy-to-buffer): Work for empty MIME part.
* mml.el (mime-to-mml): Ditto.

lisp/gnus/mm-decode.el

@@ -764,7 +764,7 @@ MIME-Version header before proceeding."
         (mb (mm-multibyte-p))
         beg)
     (goto-char (point-min))
-    (search-forward-regexp "^\n" nil t)
+    (search-forward-regexp "^\n" nil 'move) ;; There might be no body.
     (setq beg (point))
     (with-current-buffer
           (generate-new-buffer " *mm*")

lisp/gnus/mm-view.el

@@ -368,10 +368,12 @@
 	  (enriched-decode (point-min) (point-max))))
       (mm-handle-set-undisplayer
        handle
+       (if (= (point-min) (point-max))
+	   #'ignore
 	 `(lambda ()
 	    (let ((inhibit-read-only t))
 	      (delete-region ,(copy-marker (point-min) t)
-			   ,(point-max-marker))))))))
+			     ,(point-max-marker)))))))))
 
 (defun mm-insert-inline (handle text)
   "Insert TEXT inline from HANDLE."

lisp/gnus/mml.el

@@ -979,8 +979,10 @@ If HANDLES is non-nil, use it instead reparsing the buffer."
   (unless handles
     (setq handles (mm-dissect-buffer t)))
   (goto-char (point-min))
-  (search-forward "\n\n" nil t)
+  (if (search-forward "\n\n" nil 'move)
       (delete-region (point) (point-max))
+    ;; No content in the part that is the sole part of this message.
+    (insert (if (bolp) "\n" "\n\n")))
   (if (stringp (car handles))
       (mml-insert-mime handles)
     (mml-insert-mime handles t))