Just got bitten by this one in Ruby on Rails.
If you use empty? to check a hash slice that doesn’t exist you get an error, instead of the behavior of nil? which simply  reports that it is actually nil …


irb(main):004:0> test = {}
=> {}
irb(main):005:0> test = {'test' => '1'}
=> {"test"=>"1"}
irb(main):006:0> test
=> {"test"=>"1"}
irb(main):007:0> test.empty?
=> false
irb(main):008:0> test[test]
=> nil
irb(main):009:0> test['test']
=> "1"
irb(main):010:0> test['test'].empty?
=> false
irb(main):011:0> test['test'].nil?
=> false
irb(main):012:0> test['testx'].nil?
=> true
irb(main):013:0> test['testx'].empty?
NoMethodError: undefined method `empty?' for nil:NilClass
from (irb):13
irb(main):014:0>