Because progname was memoized with ||= a logger call that involved
outputting false would be nil. Example code:

  logger = Logger.new(STDOUT)
  logger.info(false)  # => nil

Perform an explicit nil check instead of ||= so that false will be output.

patched by Gavin Miller <gavingmiller@gmail.com> [Fix GH-1667]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59380 b2dd03c8-39d4-4d8f-98ff-823fe69b080e