Fix importing polygons with empty `points` attributes

This fixes a TypeError that gets logged when importing files like the following:

```xml
<?xml version="1.0"?>
<svg width="300" height="300" xmlns="http://www.w3.org/2000/svg">
 <g>
  <polygon fill="#FF0000" stroke="#000000" stroke-width="5" points=""/>
  <polygon fill="#FF0000" stroke="#000000" stroke-width="5" points="100,100 200,100 200,200 100,200"/>
 </g>
</svg>
```

The [SVG spec](http://www.w3.org/TR/SVG/shapes.html#PointsBNF) allows the list of points to have 0 coordinate pairs. I also didn't see anything that says the `points` attribute is required. In any case, I have seen SVG files saved by SVG-edit that look similar to the one above. So now if `points` is missing or empty, then we return null and the element is skipped while importing.