* lib/irb.rb: [DOC] PROMPT_I cannot be nil, patch by @hgillane [ci skip]
  Fixes documenting-ruby/ruby#37
  https://github.com/documenting-ruby/ruby/pull/37


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46970 b2dd03c8-39d4-4d8f-98ff-823fe69b080e