Fix clobbering of sockets list from constructor

Co-authored-by: Florimond Manca <florimond.manca@protonmail.com>

uvicorn/server.py

@@ -120,10 +120,10 @@ async def serve(self, sockets: Optional[List[socket.socket]] = None) -> None:
         if self._main_task is not None:
             raise RuntimeError("cannot call serve on running server")
 
-        if sockets is not None and self._sockets is not None:
-            raise RuntimeError("cannot override already provided sockets list")
-
-        self._sockets = sockets
+        if sockets is not None:
+            if self._sockets is not None:
+                raise RuntimeError("cannot override already provided sockets list")
+            self._sockets = sockets
 
         self.install_signal_handlers()
         try: