Filter netem qdiscs in JSON output for improved clarity

[FloSch62]

Nodes that have not any impairments configured will return in json:

./containerlab tools netem show -n clab-vlan-srl2 -f json 
{
  "clab-vlan-srl2": null
}

otherwise we would have duplicated interfaces for some kinds, because of clsact or other qdisc types:

{
  "clab-edge": [
    {
      "interface": "eth1",
      "delay": "10ms",
      "jitter": "30ms",
      "packet_loss": 0.05,
      "rate": 700,
      "corruption": 0.09
    },
    {
      "interface": "eth1",
      "delay": "",
      "jitter": "",
      "packet_loss": 0,
      "rate": 0,
      "corruption": 0
    },
    
    ```

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.42%. Comparing base (134b828) to head (e6d82aa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2492      +/-   ##
==========================================
- Coverage   52.47%   52.42%   -0.05%     
==========================================
  Files         172      172              
  Lines       17767    17768       +1     
==========================================
- Hits         9323     9315       -8     
- Misses       7458     7467       +9     
  Partials      986      986              

Files with missing lines	Coverage Δ
cmd/tools_netem.go	68.00% <100.00%> (-1.45%)	⬇️

... and 1 file with indirect coverage changes