habit: Fix of the fix of color for very first done of a habit

* lisp/org-habit.el (org-habit-build-graph): Check if there is a done
date at all.

Reported by Colin Baxter, see https://orgmode.org/list/877dvjve9v.fsf@yandex.com/.

See f471768a5 for the related fix.

lisp/org-habit.el

@@ -343,7 +343,8 @@ current time."
 	      (if (and in-the-past-p
 		       (not last-done-date)
 		       (not (< scheduled now)))
-		  (if (= start (car all-done-dates))
+		  (if (and all-done-dates (= (car all-done-dates) start))
+		      ;; This is the very first done of this habit.
 		      '(org-habit-ready-face . org-habit-ready-future-face)
 		    '(org-habit-clear-face . org-habit-clear-future-face))
 		(org-habit-get-faces