message: p2p: iterate eviction protection only on networks having candidates
  in ProtectEvictionCandidatesByRatio().
  Thank you to Vasil Dimov, whose suggestions during a post-merge
  discussion about PR 21261 reminded me that I had done this in
  earlier versions of the PR, e.g. commits like
diff: @@ -938,7 +938,10 @@ void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& evicti
  const size_t max_protect_by_network{total_protect_size / 2};
  size_t num_protected{0};
  - while (num_protected < max_protect_by_network) {
  + // Count the number of disadvantaged networks from which we have peers to protect.
  + auto num_networks = std::count_if(networks.begin(), networks.end(), [](const Net& n) { return n.count; });
  +
  + while (num_networks != 0 && num_protected < max_protect_by_network) {
  const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
  const size_t protect_per_network{
  std::max(disadvantaged_to_protect / networks.size(), static_cast<size_t>(1))};
  