Fix bug with invisible agenda items

Fix bug with invisible agenda items

Peter Westlake writes:

> This is quite obscure, and an odd corner case, but here it is.
>
> If you have an item which is:
>
> - a TODO
> - scheduled
> - ordered
> - blocked by a child TODO
>
> then it leaves an empty line in the agenda.
>
> Here's a test case:
> ,----
> | * Press t r on this line in the agenda
> |   SCHEDULED: <2009-09-02 Wed>
> |   :PROPERTIES:
> |   :ORDERED:  t
> |   :END:
> | *** TODO Report invisible scheduled items
> `----
>
> C-a a a will show an agenda with "Scheduled: Press t r ...".
> Press "t" on that line to add a TODO.
> Press "r" to redisplay, and see the line go blank.
>
> Peter.