Support use_nil for record values

[kaorimatz]

$ cat fluent.conf
<source>
  @type forward
</source>

<filter t1>
  @type record_modifier
  <record>
    c "#{use_nil}"
  </record>
</filter>

<match t1>
  @type record_modifier
  tag t2
  <record>
    d "#{use_nil}"
  </record>
</match>

<match t2>
  @type stdout
</match>

$ echo '{"a":"b"}' | bundle exec fluent-cat t1

Before:

$ bundle exec fluentd -c fluent.conf
...
2025-08-23 12:26:51 +0900 [error]: unexpected error error_class=NoMethodError error="undefined method `include?' for nil:NilClass"

After:

$ bundle exec fluentd -c fluent.conf
...
2025-08-23 12:27:17.577618000 +0900 t2: {"a":"b","c":null,"d":null}

[repeatedly]

Thanks for the patch!

[repeatedly]

Released v2.2.1.

[kaorimatz]

Thank you for taking the time to review and release the new version!