Fixed issue with `setAttribute()` on `SvgRenderer`

It's a simple fix actually, it just make sure that the value submited on a `setAttribute()` call is valid.

The problem was [already experienced by the Dojo toolkit folks](https://bugs.dojotoolkit.org/ticket/12229) ([diff](https://bugs.dojotoolkit.org/attachment/ticket/12229/svg.patch)).