Make NullRelation a bit more like a real relation by returning 0 for #calculate(:count)

activerecord/lib/active_record/null_relation.rb

@@ -55,8 +55,12 @@ module ActiveRecord
     end
 
     def calculate(_operation, _column_name, _options = {})
+      if _operation == :count
+        0
+      else
         nil
       end
+    end
 
     def exists?(_id = false)
       false

activerecord/test/cases/relations_test.rb

@@ -279,6 +279,7 @@ class RelationTest < ActiveRecord::TestCase
   def test_null_relation_calculations_methods
     assert_no_queries do
       assert_equal 0, Developer.none.count
+      assert_equal 0, Developer.none.calculate(:count, nil, {})
       assert_equal nil, Developer.none.calculate(:average, 'salary')
     end
   end