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, 0 deletions

diff --git a/synapse/federation/transport/server/_base.py b/synapse/federation/transport/server/_base.py

index 441ff8ce71..cba690e795 100644
--- a/synapse/federation/transport/server/_base.py
+++ b/synapse/federation/transport/server/_base.py
@@ -320,6 +320,7 @@ class BaseFederationServlet:
             if origin and whitelisted_homeserver(origin):
                 origin_context = context_from_request(request)
 
+            remote_parent_span = None
             if origin_context:
                 local_servlet_span = get_active_span()
                 # Create a span which uses the `origin_context` as a parent