Fix ISO8601Date failed to parse nil
Fix ISO8601Date failed to parse nil

Close #4019

This commit fix the regression test added in 830230f21ac971b4f4bb95b54e975988eed28360

rmosolgo/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby,nevesenin/graphql-ruby,rmosolgo/graphql-ruby,nevesenin/graphql-ruby