Fix where on polymorphic association with non Active Record object

It is a regression for #40815.

If `where` on polymorphic association with non Active Record object,
`associated_table.join_primary_key` is called without `klass` in
`AssociationQueryValue`, it is wrong usage for polymorphic association.

Always use `PolymorphicArrayValue` for polymorphic association to avoid
the issue.

Fixes #40937.

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -93,11 +93,8 @@ module ActiveRecord
             # PriceEstimate.where(estimate_of: treasure)
             associated_table = table.associated_table(key)
             if associated_table.polymorphic_association?
-              case value.is_a?(Array) ? value.first : value
-              when Base, Relation
-                value = [value] unless value.is_a?(Array)
-                klass = PolymorphicArrayValue
-              end
+              value = [value] unless value.is_a?(Array)
+              klass = PolymorphicArrayValue
             elsif associated_table.through_association?
               next associated_table.predicate_builder.expand_from_hash(
                 associated_table.primary_key => value

activerecord/lib/active_record/relation/predicate_builder/polymorphic_array_value.rb

@@ -10,10 +10,10 @@ module ActiveRecord
 
       def queries
         type_to_ids_mapping.map do |type, ids|
-          {
-            associated_table.join_foreign_type => type,
-            associated_table.join_foreign_key => ids
-          }
+          query = {}
+          query[associated_table.join_foreign_type] = type if type
+          query[associated_table.join_foreign_key] = ids
+          query
         end
       end
 
@@ -23,7 +23,7 @@ module ActiveRecord
         def type_to_ids_mapping
           default_hash = Hash.new { |hsh, key| hsh[key] = [] }
           values.each_with_object(default_hash) do |value, hash|
-            hash[klass(value).polymorphic_name] << convert_to_id(value)
+            hash[klass(value)&.polymorphic_name] << convert_to_id(value)
           end
         end
 
@@ -46,6 +46,8 @@ module ActiveRecord
             value._read_attribute(primary_key(value))
           when Relation
             value.select(primary_key(value))
+          else
+            value
           end
         end
     end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -44,6 +44,11 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_equal [authors(:david)], Author.where(owned_essay: essays(:david_modest_proposal))
   end
 
+  def test_where_on_polymorphic_association_with_nil
+    assert_equal comments(:greetings), Comment.where(author: nil).first
+    assert_equal comments(:greetings), Comment.where(author: [nil]).first
+  end
+
   def test_assigning_belongs_to_on_destroyed_object
     client = Client.create!(name: "Client")
     client.destroy!