Fix for custom tag rendering

test/unit/tag_unit_test.rb

@@ -20,4 +20,13 @@ class TagUnitTest < Minitest::Test
     tag = Tag.parse("some_tag", "", Tokenizer.new(""), ParseContext.new)
     assert_equal('some_tag', tag.tag_name)
   end
+
+  class CustomTag < Liquid::Tag
+    def render(_context); end
+  end
+
+  def test_tag_render_to_output_buffer_nil_value
+    custom_tag = CustomTag.parse("some_tag", "", Tokenizer.new(""), ParseContext.new)
+    assert_equal('some string', custom_tag.render_to_output_buffer(Context.new, "some string"))
+  end
 end