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/

app/controllers/users_controller.rb

@@ -67,12 +67,8 @@ class UsersController < ApplicationController
   end
 
   def calendar_activities
-    @calendar_date = Date.parse(params[:date]) rescue nil
-    @events = []
-
-    if @calendar_date
+    @calendar_date = Date.parse(params[:date]) rescue Date.today
     @events = contributions_calendar.events_by_date(@calendar_date)
-    end
 
     render 'calendar_activities', layout: false
   end