Correctly check for show_wirenumbers default of None

[gopiballava]

If show_wirenumbers is omitted from a cable section, its value will be None. In that case, we want to choose a default based on whether this is a bundle or not.

If the user did specify show_wirenumbers, then its value will be True or False, and we want to respect that whether it's a bundle or not.

[kvid]

I recommend accepting this PR based on a simple test with this YAML input:

cables:
  MyCable:
    colors: ['BN', 'RD', 'YE']
    show_wirenumbers: False

The left diagram shows that the result without this PR doesn't respect the show_wirenumbers: False for non-bundle cables, and the right diagram shows the result from the same YAML input after applying this PR:

I have also verified that none of the generated example files change after applying this PR:

git checkout -b dev-built origin/dev
python build_examples.py
git add ../../{examples,tutorial}/*
git commit
python build_examples.py restore
git checkout gopiballava-fix-show_wirenumbers
python build_examples.py
python build_examples.py -b dev-built -c compare
python build_examples.py restore