`read` sets `vp_pen_width` even when it is not defined in the SVG

[abey79]

<svg xmlns="http://www.w3.org/2000/svg" width="650" height="650">
    <circle cx="150" cy="150" r="100" stroke="red" fill="green"/>
    <rect x="400" y="200" width="200" height="400" stroke="blue" fill="green" />
    <path d="M250,600 l-200,0 l0,-200 z" stroke="blue" fill="green" />
</svg>

$ vpype read example.svg proplist --layer 1 show
listing 2 properties for layer 1
  svg_fill: (str) green
  vp_pen_width: (float) 1.0

[abey79]

The issue here is inconsistent behaviour:

• heterogenous attributes --> default vpype value (blue, 0.25mm)
• attributes missing altogether --> default CSS value (black, 1px)