popular posts: fix exception when given blank date param.

Flexbooru calls this endpoint with a blank date param.

app/controllers/explore/posts_controller.rb

@@ -38,7 +38,7 @@ module Explore
     private
 
     def parse_date(params)
-      date = params[:date] ? Date.parse(params[:date]) : Time.zone.today
+      date = params[:date].present? ? Date.parse(params[:date]) : Time.zone.today
       scale = params[:scale].in?(["day", "week", "month"]) ? params[:scale] : "day"
       min_date = date.send("beginning_of_#{scale}")
       max_date = date.send("next_#{scale}").send("beginning_of_#{scale}")

test/functional/explore/posts_controller_test.rb

@@ -12,6 +12,11 @@ module Explore
           get popular_explore_posts_path
           assert_response :success
         end
+
+         should "work with a blank date" do
+          get popular_explore_posts_path(date: "")
+          assert_response :success
+         end
       end
 
       context "#curated" do