conf: avoid quadratic complexity

conf: avoid quadratic complexity

Ticket: 6878

Follow up on 15649424a76d01eb332d85620ffc4956d4f3d9be

When adding many sequence nodes, either from start or scalar event

We add "sequence nodes" whose name is an integer cf sequence_node_name
and then run ConfNodeLookupChild to see if it had been already set
(from the command line cf comment in the code)
And ConfNodeLookupChild iterates the whole linked list...

1. We add node 1
2. To add node 2, we check if node 1 equals this new node
3. To add node 3, we check if nodes 1, or 2 equals this new node's name
And so on...

This commits avoids these checks ig the list is empty at the beginning