[add] null client handling

Author: i4004

src/Etcd.Microsoft.Extensions.Configuration/Client/EtcdKeyValueClient.cs

@@ -131,7 +131,7 @@ public void Dispose()
 		if (_unwatchOnDispose)
 			StopWatchAll();
 
-		_client.Dispose();
+		_client?.Dispose();
 	}
 
 	/// <summary>