Speed up Hash#transform_values when empty

We're calling this function on an empty hash as part of copying the
attribute set during dirty checking initialization. The new structure
caused a performance regression on loading records from the database.
This causes `User.all.to_a` to perform about 10% faster w/ 10k records.

Calculating -------------------------------------
   User.all - master     9.000  i/100ms
   User.all - sg-fix-ar-regression
                         8.000  i/100ms
-------------------------------------------------
   User.all - master     81.236  (± 7.4%) i/s -    405.000
   User.all - sg-fix-ar-regression
                         89.716  (± 7.8%) i/s -    448.000

activesupport/lib/active_support/core_ext/hash/transform_values.rb

@@ -6,6 +6,7 @@ class Hash
   #   # => { a: 2, b: 4, c: 6 }
   def transform_values
     return enum_for(:transform_values) unless block_given?
+    return {} if empty?
     result = self.class.new
     each do |key, value|
       result[key] = yield(value)