Bug: undefined srcset with use-picture

Heyho :wave:

The following code will result in an `undefined undefined` value for the `srcset` of the `source` slot.
Using a string instead (`:srcset="'https://googlechrome.github.io/samples/picture-element/images/butterfly.webp'"`) is a workaround to avoid that issue.

```vue



```

CSB: https://codesandbox.io/s/3097oj4rlp