Support base branch that doesn't have any required check

Right now the bot assumes that required check always exist. When it doesn't the bot will probably won't continue process the PR.

The bot should merge the PR when there's no required check and it's up to date.

No file or test is named. Start by tracing the action's pull-request status handling and how it determines required checks; verify the behavior for an up-to-date PR whose base branch defines no required checks, and confirm that the PR can proceed to merge.

Written by the indexing model from the issue text.