Migrate out_null plugin to v0.14 API

[okkez]

No description provided.

[cosmo0920]

I think that adding the following patch into test driver and test assertion is better:

diff --git a/lib/fluent/test/driver/base.rb b/lib/fluent/test/driver/base.rb
index 229acc1..e52b776 100644
--- a/lib/fluent/test/driver/base.rb
+++ b/lib/fluent/test/driver/base.rb
@@ -106,6 +106,7 @@ module Fluent
         end

         def events(tag: nil)
+          return [] if @event_streams.nil?
           selected = @event_streams.select{|e| tag.nil? ? true : e.tag == tag }
           if block_given?
             selected.each do |e|
diff --git a/test/plugin/test_out_null.rb b/test/plugin/test_out_null.rb
index a3f3815..ab47021 100644
--- a/test/plugin/test_out_null.rb
+++ b/test/plugin/test_out_null.rb
@@ -24,5 +24,6 @@ class NullOutputTest < Test::Unit::TestCase
         d.feed("test", Fluent::EventTime.now, {"test" => "null"})
       end
     end
+    assert_equal([], d.events)
   end
 end

[okkez]

Thanks I've added your patch @cosmo0920

[tagomoris]

Looks very good to me. Thank you!