In UsersController#calendar_activities, when Date isn't parsable, fallback to Date.today

For some reason, GoogleBot accesses `/u/:username/calendar_activities` without
a `:date` param, but then the view was trying to call `#to_s(:short)` which
doesn't exist on `nil`, leading to the following Sentry report:
https://sentry.gitlap.com/gitlab/gitlabcom/issues/1182/

See merge request !3074