group_by_day returns nil when dates.keys.first is not a String

[VitaliyAdamkov]

ruby 2.3.1
rails 4.2
upmin 0.1.3

Started GET "/admin" for 127.0.0.1 at 2018-01-10 12:22:16 +0200
Processing by Upmin::DashboardController#index as HTML
User Load (2.1ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT 1 [["id", 1]]
Pair Load (1.1ms) SELECT "pairs".* FROM "pairs" ORDER BY date(created_at) DESC LIMIT 1
Pair Load (0.9ms) SELECT "pairs".* FROM "pairs" ORDER BY date(created_at) ASC LIMIT 1
CACHE (0.0ms) SELECT "pairs".* FROM "pairs" ORDER BY date(created_at) DESC LIMIT 1
CACHE (0.0ms) SELECT "pairs".* FROM "pairs" ORDER BY date(created_at) ASC LIMIT 1
(0.7ms) SELECT COUNT(*) AS count_all, date(created_at) AS date_created_at FROM "pairs" WHERE ("pairs"."created_at" IS NOT NULL) GROUP BY date(created_at) ORDER BY date(created_at) ASC
Rendered /home/adamkov/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/upmin-admin-0.1.3/app/views/upmin/dashboard/_chart.html.haml (14.8ms)
Rendered /home/adamkov/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/upmin-admin-0.1.3/app/views/upmin/dashboard/index.html.haml within layouts/upmin/application (15.8ms)
Completed 401 Unauthorized in 33ms (ActiveRecord: 5.5ms)
NoMethodError - undefined method `each_with_object' for nil:NilClass

it works like a charm when it will return [] in that case

##181