Let `org-agenda-time-grid' control if the grid is displayed.

Let `org-agenda-time-grid' control if the grid is displayed.

* org-agenda.el (org-agenda-list): Ensure that the
list returned by `org-agenda-add-time-grid-maybe' is appended
to `rtnall' before checking if the latter is emtpy.

In the case where rtnall is empty (no item for current day), we do not
call org-agenda-add-time-grid-maybe.  This seems bogus because that
function is already computing whether the time grid must be included,
and does so according to the user's preferences.

In particular, the `org-agenda-time-grid' variable has a
'require-timed' parameter controlling the visibility of the time grid.

So, this patch :

 - removes the premature check for rtnall being empty,
 - inconditionally calls org-agenda-add-time-grid-maybe,
 - and finally checks the emptiness of the resulting list
   before pretty printing.

TINYCHANGE