Don't create marker if target is entire file.
* org-agenda.el (org-agenda-bulk-action): Don't create marker for
position if target is entire file.
If the target of a bulk refile operation is the entire file,
`org-refile-get-location' returns nil for the refile position.
Creating a marker for the target file's buffer at position nil returns
a marker that points nowhere (Cf. GNU Emacs Lisp Reference Manual,
31.6). `org-refile' adds headings to level 1 if the target position
for the target file is nil -- and hence a marker that points nowhere
is not nil, tries to jump to nowhere.