fix missing block when there is no failure
# What does this PR do?

Currently, when doc test has no failure, we don't see anything other than `🤗 Results of the doc tests.` in slack report.
This is because the block defined in `no_failures` property is not included in `def payload`, as the condition `self.no_failures == 0` should be `self.n_failures == 0`.
@amyeroberts  request you as a reviewer just in case you want to take a look.<|||||>_The documentation is not available anymore as the PR was closed or merged._