Restore printing sentinel for log_record.request (#19172)

This was unintentionally changed in
https://github.com/element-hq/synapse/pull/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
https://github.com/element-hq/synapse/issues/19165


### Reproduction strategy

1. Configure Synapse with
[logging](https://github.com/element-hq/synapse/blob/df802882bb2ec7d52d5c064c20531fe5a5b263b1/docs/sample_log_config.yaml)
1. Start Synapse: `poetry run synapse_homeserver --config-path
homeserver.yaml`
1. Notice the `asyncio - 64 - DEBUG - - Using selector: EpollSelector`
log line (notice empty string `- -`)
1. With this PR, the log line will be `asyncio - 64 - DEBUG - sentinel -
Using selector: EpollSelector` (notice `sentinel`)

changelog.d/19172.misc

@@ -0,0 +1 @@
+Restore printing `sentinel` for the log record `request` when no logcontext is active.

synapse/logging/context.py

@@ -619,19 +619,24 @@ class LoggingContextFilter(logging.Filter):
             True to include the record in the log output.
         """
         context = current_context()
-        record.request = self._default_request
-
-        # Avoid overwriting an existing `server_name` on the record. This is running in
-        # the context of a global log record filter so there may be 3rd-party code that
-        # adds their own `server_name` and we don't want to interfere with that
-        # (clobber).
-        if not hasattr(record, "server_name"):
-            record.server_name = "unknown_server_from_no_logcontext"
-
-        # context should never be None, but if it somehow ends up being, then
-        # we end up in a death spiral of infinite loops, so let's check, for
+        # type-ignore: `context` should never be `None`, but if it somehow ends up
+        # being, then we end up in a death spiral of infinite loops, so let's check, for
         # robustness' sake.
-        if context is not None:
+        #
+        # Add some default values to avoid log formatting errors.
+        if context is None:
+            record.request = self._default_request  # type: ignore[unreachable]
+
+            # Avoid overwriting an existing `server_name` on the record. This is running in
+            # the context of a global log record filter so there may be 3rd-party code that
+            # adds their own `server_name` and we don't want to interfere with that
+            # (clobber).
+            if not hasattr(record, "server_name"):
+                record.server_name = "unknown_server_from_no_logcontext"
+
+        # Otherwise, in the normal, expected case, fill in the log record attributes
+        # from the logcontext.
+        else:
 
             def safe_set(attr: str, value: Any) -> None:
                 """