uploads: fix error when source doesn't have any images.

Fix an error when trying to upload a source that doesn't have any
images, for example a Twitter post with no images.

app/models/upload.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class Upload < ApplicationRecord
+  class Error < StandardError; end
+
   attr_accessor :file
 
   belongs_to :uploader, class_name: "User"
@@ -96,14 +98,19 @@ class Upload < ApplicationRecord
     elsif source.present?
       strategy = Sources::Strategies.find(source, referer_url)
       page_url = strategy.page_url
+      image_urls = strategy.image_urls
 
-      upload_media_assets = strategy.image_urls.map do |image_url|
+      if image_urls.empty?
+        raise Error, "#{source} doesn't contain any images"
+      end
+
+      upload_media_assets = image_urls.map do |image_url|
         UploadMediaAsset.new(source_url: image_url, page_url: page_url, media_asset: nil)
       end
 
       update!(upload_media_assets: upload_media_assets, media_asset_count: upload_media_assets.size)
     else
-      raise "No file or source given" # Should never happen
+      raise Error, "No file or source given" # Should never happen
     end
   rescue Exception => e
     update!(status: "error", error: e.message)

test/functional/uploads_controller_test.rb

@@ -258,6 +258,16 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
         end
       end
 
+      context "for a source that doesn't contain any images" do
+        should "fail" do
+          create_upload!("https://twitter.com/danboorubot/status/923612084616577024", user: @user)
+
+          assert_response 201
+          assert_equal(true, Upload.last.is_errored?)
+          assert_match("doesn't contain any images", Upload.last.error)
+        end
+      end
+
       should "work for a source URL containing unicode characters" do
         source1 = "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg?one=東方&two=a%20b"
         source2 = "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg?one=%E6%9D%B1%E6%96%B9&two=a%20b"