Use nil for non-existing files rather than 0

app/models/ci/build.rb

@@ -382,7 +382,9 @@ module Ci
     private
 
     def update_artifacts_size
-      self.artifacts_size = artifacts_file.size
+      self.artifacts_size = if artifacts_file.exists?
+                              artifacts_file.size
+                            end
     end
 
     def erase_trace!

spec/requests/ci/api/builds_spec.rb

@@ -486,7 +486,7 @@ describe Ci::API::API do
           expect(response).to have_http_status(200)
           expect(build.artifacts_file.exists?).to be_falsy
           expect(build.artifacts_metadata.exists?).to be_falsy
-          expect(build.artifacts_size).to eq(0)
+          expect(build.artifacts_size).to be_nil
         end
       end