Empty section is valid in most case

Signed-off-by: Kenji Okimoto <okimoto@clear-code.com>

app/models/concerns/fluentd/setting/section_validator.rb

@@ -10,6 +10,7 @@ class Fluentd
       def validate_sections
         self._section_params.each do |name, sections|
           sections.each do |section|
+            next if section.attributes.values.all?(&:blank?)
             if section.invalid?
               errors.add(name, :invalid, message: section.errors.full_messages)
             end