fix: Fix lifespan app handler not working

It was noticed that shutdown handlers for implementations are not
executed.

Investigation reveals that lifespan logic is not working. When server
starts, we see:

```
ASGI 'lifespan' protocol appears unsupported.
```

This is because uvicorn calls to app with a missing path on startup,
(perhaps to detect that the protocol is working?) and our
TracingMiddleware was not ready for that, resulting in an exception and
lifespan logic disabled.

Fixes #188

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/distribution/server/server.py

@@ -215,7 +215,7 @@ class TracingMiddleware:
         self.app = app
 
     async def __call__(self, scope, receive, send):
-        path = scope["path"]
+        path = scope.get("path", "")
         await start_trace(path, {"__location__": "server"})
         try:
             return await self.app(scope, receive, send)