RUBY-1171 Check explicitly that user name is not set in the selector when nil
RUBY-1171 Check explicitly that user name is not set in the selector when nil

jonhyman/mongo-ruby-driver,saghm/mongo-ruby-driver,estolfo/mongo-ruby-driver,saghm/mongo-ruby-driver,mongodb/mongo-ruby-driver,mongodb/mongo-ruby-driver,estolfo/mongo-ruby-driver,mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver,kay-kim/mongo-ruby-driver,kay-kim/mongo-ruby-driver