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

lisp/org-agenda.el

@@ -3895,10 +3895,10 @@ given in `org-agenda-start-on-weekday'."
 	    (put-text-property s (1- (point)) 'org-day-cnt day-cnt)
 	    (when todayp
 	      (put-text-property s (1- (point)) 'org-today t))
+	    (setq rtnall
+		  (org-agenda-add-time-grid-maybe rtnall ndays todayp))
 	    (if rtnall (insert ;; all entries
-			(org-finalize-agenda-entries
-			 (org-agenda-add-time-grid-maybe
-			  rtnall ndays todayp))
+			(org-finalize-agenda-entries rtnall)
 			"\n"))
 	    (put-text-property s (1- (point)) 'day d)
 	    (put-text-property s (1- (point)) 'org-day-cnt day-cnt))))