reports: fix error when report is empty.

Fix an exception when a report is empty, for example when performing a
tag search that returns no results:

* https://betabooru.donmai.us/reports/posts?search[group]=uploader&search[tags]=does_not_exist

test/functional/reports_controller_test.rb

@@ -26,6 +26,18 @@ class ReportsControllerTest < ActionDispatch::IntegrationTest
           assert_response :success
         end
 
+        should "work with the period and group params" do
+          get report_path("posts", search: { period: "month", group: "uploader" })
+
+          assert_response :success
+        end
+
+        should "work with the group param when the dataset is empty" do
+          get report_path("posts", search: { tags: "does_not_exist", group: "uploader" })
+
+          assert_response :success
+        end
+
         should "work when filtering by a nested association" do
           get report_path("posts", search: { uploader: { name: @post.uploader.name }})