Restore printing sentinel for log_record.request

[MadLittleMods]

Restore printing sentinel for log_record.request

This was unintentionally changed in #19068.

There is no real bug here. Without this PR, we just printed an empty string for the sentinel logcontext whereas the prior art behavior was to print sentinel which this PR restores.

Found while staring at the logs in #19165

Reproduction strategy

1. Configure Synapse with logging
2. Start Synapse: poetry run synapse_homeserver --config-path homeserver.yaml
3. Notice the asyncio - 64 - DEBUG - - Using selector: EpollSelector log line (notice empty string - -)
4. With this PR, the log line will be asyncio - 64 - DEBUG - sentinel - Using selector: EpollSelector (notice sentinel)

Pull Request Checklist

• Pull request is based on the develop branch
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• Code style is correct (run the linters)

[MadLittleMods]

To explain the bug, we would default record.request to an empty string here, and then when we later used safe_set("request", str(context)) below, it would see our default value and not clobber it to sentinel.

This only results in an empty string getting printed whereas the prior art was to print sentinel which this PR restores.

[MadLittleMods]

Thanks for the review @reivilibre 🐆