Fix: user email should be null instead of empty string

db/migrate/20170131164224_user_email_can_be_null.rb

@@ -0,0 +1,15 @@
+class UserEmailCanBeNull < ActiveRecord::Migration
+  def up
+    change_table :users do |t|
+      t.change :email, :string, default: nil, null: true
+    end
+
+    User.where(email: '').update_all(email: nil)
+  end
+
+  def down
+    change_table :users do |t|
+      t.change :email, :string, default: '', null: false
+    end
+  end
+end