gelbooru: fix exception when fetching data for deleted post.

Fix exceptions when fetching deleted or nonexistent posts from Gelbooru. The Gelbooru API doesn't
return any data for deleted posts.

app/logical/source/extractor/gelbooru.rb

@@ -28,7 +28,7 @@ module Source
       end
 
       def gelbooru_tags
-        return [] if api_response.nil?
+        return [] if api_response.blank?
 
         tags = api_response[:tags].split + ["rating:#{api_response[:rating]}"]
         tags.map do |tag|
@@ -40,6 +40,10 @@ module Source
         sub_extractor&.tags.to_a
       end
 
+      def other_names
+        sub_extractor&.other_names.to_a
+      end
+
       def post_id
         parsed_url.post_id || parsed_referer&.post_id || post_id_from_md5
       end
@@ -50,12 +54,12 @@ module Source
       end
 
       memoize def api_response
-        return nil unless api_url.present?
+        return {} unless api_url.present?
 
         response = http.cache(1.minute).get(api_url)
-        return nil unless response.status == 200
+        return {} unless response.status == 200
 
-        response.parse["post"].first.with_indifferent_access
+        response.parse["post"]&.first.to_h.with_indifferent_access
       end
 
       memoize def post_id_from_md5

test/unit/sources/gelbooru_test.rb

@@ -68,6 +68,30 @@ module Sources
       )
     end
 
+    context "A deleted Gelbooru post" do
+      strategy_should_work(
+        "https://gelbooru.com/index.php?page=post&s=list&md5=9d06e876937d46eeda7a5e0ca52f63a8",
+        image_urls: [],
+        artist_name: nil,
+        profile_url: nil,
+        tags: %w[],
+        artist_commentary_title: nil,
+        artist_commentary_desc: nil,
+      )
+    end
+
+    context "A nonexistent Gelbooru post" do
+      strategy_should_work(
+        "https://gelbooru.com/index.php?page=post&s=list&md5=ffffffffffffffffffffffffffffffff",
+        image_urls: [],
+        artist_name: nil,
+        profile_url: nil,
+        tags: %w[],
+        artist_commentary_title: nil,
+        artist_commentary_desc: nil,
+      )
+    end
+
     should "normalize gelbooru links" do
       source1 = "https://gelbooru.com//images/ee/5c/ee5c9a69db9602c95debdb9b98fb3e3e.jpeg"
       source2 = "http://simg.gelbooru.com//images/2003/edd1d2b3881cf70c3acf540780507531.png"