[bug] Asking for an equivalent gauge will fail if no gauge is specified

[kvid]

Such a small YAML input example below will fail with the exception "AttributeError: 'NoneType' object has no attribute 'upper'"

cables:
  W1:
    colors: [BK, RD]
    show_equiv: true
connections:
  - - W1

I have provided a fix in #498.

My context:

$ wireviz -V && dot -V && python -V && lsb_release -a

WireViz 0.4.1
dot - graphviz version 2.43.0 (0)
Python 3.12.3
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 24.04.4 LTS
Release:        24.04
Codename:       noble