Document that Net::IMAP#fetch will return nil instead of an emtpy array.

* lib/net/imap.rb (Net::IMAP#fetch): [DOC] Document that
  Net::IMAP#fetch will return nil instead of an emtpy array.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48207 b2dd03c8-39d4-4d8f-98ff-823fe69b080e