Configuration should allow empty peer url list

[SatpalSandhu61]

When creating a network of tessera nodes using tools such as Quorum Maker, we need to allow the very first node to be created with no peers. The first node will then become part of a network by means of the other nodes having the first node in their url list.

For this reason we should allow the peer url list to be empty (currently, at least one node must be specified).

See associated issue # 577.