Fix error when parsing empty SVG document.

This fixes an error (`'undefined' is not a function (evaluating 'doc.selectNodes('//*[name(.)!="svg"]')')`) that was being thrown when trying to parse an SVG document with no shapes:

```xml
<?xml version="1.0"?>
<svg viewBox="0 0 120 120" version="1.1"
  xmlns="http://www.w3.org/2000/svg">
</svg>
```