MONGOID-5029 #empty method should use an #exists? rather than a #count query (#5029)

* Fixes MONGOID-5029
- `#empty?` method should use an exists rather than a count query.
- `#empty?` and `#any?` should short-circuit avoid making a query if _added is set
- Make `#empty?` congruent with `#any?` code-wide

* #any? is exactly the inverse of #empty?

* Specs for empty?

* Add spec for any

* Fix all feedback from oleg

* adjust the tests since there are now fewer queries

Co-authored-by: shields <shields@tablecheck.com>
Co-authored-by: Oleg Pudeyev <code@olegp.name>

Author: johnnyshields

lib/mongoid/association/referenced/has_many/enumerable.rb

@@ -190,9 +190,9 @@ def each
           # @return [ true, false ] If the enumerable is empty.
           def empty?
             if _loaded?
-              in_memory.count == 0
+              in_memory.empty?
             else
-              _unloaded.count + _added.count == 0
+              _added.empty? && !_unloaded.exists?
             end
           end
 
@@ -223,11 +223,7 @@ def empty?
           def any?(*args)
             return super if args.any? || block_given?
 
-            if _loaded?
-              in_memory.length > 0
-            else
-              _unloaded.exists? || _added.length > 0
-            end
+            !empty?
           end
 
           # Get the first document in the enumerable. Will check the persisted