rand stream starts with `0`

Since #1278 , when `rand` is sampled at the start (cycle 0), it always returns 0.

Now, every number is as likely as any other, but this seems a little bit implausible. It means that for example

```js
sound(wchooseCycles(["bd", 0.0001], ["sd", 1000000000]))
```

starts with a very unlikely kick.

Not sure how best to fix this, maybe just return 0.5 if the onset is 0?