mu4e: fix sending emails with no body

mu4e/mu4e-draft.el

@@ -302,7 +302,12 @@ separator is never written to the message file. Also see
       ;; search for the first empty line
       (goto-char (point-min))
       (if (search-forward-regexp "^$" nil t)
+          (progn
             (replace-match sepa)
+            ;; `message-narrow-to-headers` searches for a `mail-header-separator` followed by a new
+            ;; line. Therefore, we must insert a newline if on the last line of the buffer.
+            (when (= (point) (point-max))
+              (insert "\n")))
           (progn ;; no empty line? then prepend one
             (goto-char (point-max))
             (insert "\n" sepa))))))