Hopefully fix problem when OTEL not installed with non recording span

```
Traceback (most recent call last):
  File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 306, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 512, in _async_render
    callback_return = await raw_callback_return
  File "/usr/local/lib/python3.9/site-packages/synapse/federation/transport/server/_base.py", line 357, in new_func
    remote_parent_span = create_non_recording_span()
  File "/usr/local/lib/python3.9/site-packages/synapse/logging/tracing.py", line 502, in create_non_recording_span
    return opentelemetry.trace.NonRecordingSpan(
AttributeError: 'NoneType' object has no attribute 'trace'
```

1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/metrics/background_process_metrics.py b/synapse/metrics/background_process_metrics.py

index 59d956dd9d..ad639fe223 100644
--- a/synapse/metrics/background_process_metrics.py
+++ b/synapse/metrics/background_process_metrics.py
@@ -236,7 +236,7 @@ def run_as_background_process(
                         attributes={SynapseTags.REQUEST_ID: str(context)},
                     )
                 else:
-                    ctx = nullcontext()  # type: ignore[assignment]
+                    ctx = nullcontext()
                 with ctx:
                     return await func(*args, **kwargs)
             except Exception: