Fix `org-store-log-note'
* lisp/org.el (org-store-log-note): Ignore empty log buffers.
The regression was introduced in
f776e65373fa135fffb51e201698823fbfb3865b. From then, (org-split-string
"") no longer returns nil.
Suggested-by: Dale Sedivec <dale@codefu.org>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-08/msg00013.html>