Add Symbol#blank? to skip respond_to?(:empty?)

Any classes that don't implement #blank? will fall back to Object's
implementation, which checks whether #empty? is defined before either
using #empty? or implicit truthiness. Since checking whether #empty? is
defined is expensive, some core classes (Array/Hash) alias #blank?
directly to #empty? to skip the respond_to? check.

This commit applies the alias optimization to Symbol. #blank? is called
on Symbols for many Active Record query methods (select, includes,
group, order, joins, etc.) so it seems reasonable to define the #blank?
alias on Symbol as well.

activesupport/lib/active_support/core_ext/object/blank.rb

@@ -100,6 +100,14 @@ class Hash
   alias_method :blank?, :empty?
 end
 
+class Symbol
+  # A Symbol is blank if it's empty:
+  #
+  #   :''.blank?     # => true
+  #   :symbol.blank? # => false
+  alias_method :blank?, :empty?
+end
+
 class String
   BLANK_RE = /\A[[:space:]]*\z/
   ENCODED_BLANKS = Concurrent::Map.new do |h, enc|