Adding null patch entry to cfbot/CommitFest

I have observed cases where a cfbot entry fails without clear reason
[1]. Even if the patch just modifies comments, it has failed in some
cfbot test. In this case we could easily guess that master branch
might have problems at the time when the tests performed. But if a
patch modifies some codes, it's not that easy because it is possible
that the test reveals hidden defects of the patch or maybe not. To
make reviewer's job easier, I would like to propose cfbot/CommitFest
have "null patch entry", which is a dummy entry without any patch,
just for testing healthiness of master branch.

One might think adding new buildfarm animals could do the same. But I
think adding to cfbot/CommitFest is better because:

- Test timing. Cfbot possibly runs the null patch entry test everyday
so that one can easily compare a particular patch's test results with
the null patch test results easily.

- Test method. By letting cfbot run the test, one can easily compare
test results because the test method is identical.

[1] https://www.postgresql.org/message-id/20250422.111139.1502127917165838403.ishii%40postgresql.org

Best regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp