Work when missing xmlns="http://www.w3.org/2000/svg"

An svg that doesn't declare `xmlns="http://www.w3.org/2000/svg"` fails picosvg, for example:

```svg

```

results in:

```shell
ValueError: Unable to convert to picosvg: BadElement: /svg[0]/g[1],BadElement: /svg[0]/g[1]/path[0],BadElement: /svg[0]/g[1]/path[1]
```

Start by running the provided SVG through picosvg and tracing the conversion path that rejects the missing namespace. Done means this SVG is accepted and converted without the reported BadElement errors; no source file or test is named in the issue, so locate the relevant namespace handling and add coverage there.

Written by the indexing model from the issue text.