Fix 500 errors on searching invalid URLs (#3613)

app/services/fetch_remote_resource_service.rb

@@ -5,7 +5,7 @@ class FetchRemoteResourceService < BaseService
 
   def call(url)
     @url = url
-    process_url unless atom_url.nil?
+    process_url unless fetched_atom_feed.nil?
   end
 
   private

spec/services/fetch_remote_resource_service_spec.rb

@@ -10,7 +10,7 @@ describe FetchRemoteResourceService do
       url = 'http://example.com/missing-atom'
       service = double
       allow(FetchAtomService).to receive(:new).and_return service
-      allow(service).to receive(:call).with(url).and_return([nil, 'body'])
+      allow(service).to receive(:call).with(url).and_return(nil)
 
       result = subject.call(url)
       expect(result).to be_nil