snippet.rb

class Object
  # nil if false, else self
  def only_if(&block)
    self if self.instance_eval(&block)
  end
end

puts ''.only_if { length > 0 } || 'previously blank'
# => 'previously blank'

puts 'not blank'.only_if { length > 0 } || 'previously blank'
# => 'not blank'