Hi,

Previously, when `org-agenda-files' was nil, and `org-notify-start' had
been called (org-notify running), I got the following error every time
`org-notify-process' would run.

Wrong type argument: stringp, nil

This patch fixes this so `org-notify' won't throw errors if no agenda
files are present.

Please let me know if you spot any issues. I thought about adding a
one-time message warning the user, would this be a good idea (or should
we just stay out of their way)? I would be happy to implement it if that
would be preferable.

Thanks,
-Jay