Skip triggering "instantiation.active_record" notification when there are no records

[fatkodima]

When loading association records, Active Record unnecessary and confusingly triggers an instantiation.active_record notification even when there are no records.

This is not happening when loading records from a regular empty relation

rails/activerecord/lib/active_record/relation.rb

Lines 1441 to 1442 in 4fa5681

	def instantiate_records(rows, &block)
	return [].freeze if rows.empty?

I found this bug when subscribed to that event and got a strange payload with 0 count:

ActiveSupport::Notifications.subscribe("instantiation.active_record") do |*, payload|
  puts payload # => {:record_count=>0, :class_name=>"User"}
end

[skipkayhil]

@fatkodima this test appears to be flaking: https://buildkite.com/rails/rails/builds/109040#01909fca-c1b6-48a1-9092-8e1ea1fa3567