Ruby 判断是否空 empty? nil?

empty? 对象的值是否为空
nil? 对象本身是否为空

rb(main):015:0> ‘‘.empty?
=> true
irb(main):016:0> ‘ ‘.empty?
=> false
irb(main):017:0> "".empty?
=> true
irb(main):018:0> " ".empty?
=> false
irb(main):019:0> nil.empty?
NoMethodError: undefined method `empty?‘ for nil:NilClass
    from (irb):19
    from /usr/bin/irb:12:in `<main>‘
irb(main):020:0> nil.nil?
=> true
irb(main):021:0> ‘‘.nil?
=> false
irb(main):022:0> ‘ ‘.nil?
=> false
irb(main):023:0>