set finished false on planting factory

some code+tests doesn't cope with nil

spec/factories/planting.rb

@@ -6,7 +6,7 @@ FactoryBot.define do
     planted_at { Time.zone.local(2014, 7, 30) }
     quantity 33
     description "This is a *really* good plant."
-    finished nil
+    finished false
     finished_at nil
 
     factory :seed_planting do