Fix Time fields bug which occurs when Time value is nil

Created by: rnitta

When there are null-allowed Time column on DB, https://github.com/thoughtbot/administrate/blob/8d86f9f0833aed9394ef1bd1f0edef1de4e89b83/app/views/fields/time/_index.html.erb#L17

This code can raise error, becauseundefined method 'strftime' for nil:NilClass if field.data is nil

so I add code to check if time value is nill and some specs

please check it.

and, CircleCi tests failed due to dependencies vulnerability. please check this, too. https://github.com/thoughtbot/administrate/pull/1225