Fix #5082: NoMethodError when searching an old-style dead fanbox url in artist urls.

This API call:

    # profile: https://www.pixiv.net/fanbox/creator/40684196
    curl -H "Origin: https://fanbox.cc" "https://api.fanbox.cc/creator.get?userId=40684196"

returns `{ "body": nil }` when the artist is deleted. We didn't expect `body` to be nil.

Also fix it so that `profile_url` returns the `https://www.pixiv.net/fanbox/creator/40684196`
URL if we can't get the `https://<username>.fanbox.cc` URL, usually because the API call failed
because the artist is deleted.

test/unit/sources/fanbox_test.rb

@@ -135,6 +135,15 @@ module Sources
       end
     end
 
+    context "A deleted /fanbox/creator/:id profile url" do
+      should "not raise an exception" do
+        source = Source::Extractor.find("https://www.pixiv.net/fanbox/creator/40684196")
+
+        assert_equal("https://www.pixiv.net/fanbox/creator/40684196", source.profile_url)
+        assert_nothing_raised { source.to_h }
+      end
+    end
+
     context "generating page urls" do
       should "convert cover images to the profile url" do
         cover = "https://pixiv.pximg.net/c/400x400_90_a2_g5/fanbox/public/images/creator/1566167/profile/Ix6bnJmTaOAFZhXHLbWyIY1e.jpeg"