fix(gateway): don't clobber asyncua's default application_uri with None

Only set application_uri when server_uri is configured; otherwise
asyncua's sane default ('urn:freeopcua:client') stood, but the code
explicitly overwrote it with None. Also fixes mypy strict-optional
error caught by the quality gate.

ingestor/connection/pool.py

@@ -75,7 +75,9 @@ class OpcConnectionPool(SientiaMonitoring):
 
     async def _connect(self, server_cfg: dict) -> Client:
         client = Client(server_cfg['url'], timeout=10)
-        client.application_uri = server_cfg.get('server_uri')
+        server_uri = server_cfg.get('server_uri')
+        if server_uri:
+            client.application_uri = server_uri
         if server_cfg.get('use_security'):
             cert, key = await self.cert_provider.fetch()
             await apply_security(client, server_cfg.get('server_uri'), cert, key, None)