CDC's SinkIOWorkers setting does not respect negative values

The description of this setting says

```
"changefeed.sink_io_workers",
"the number of workers used by changefeeds when sending requests to the sink "+
"(currently the batching versions of webhook, pubsub, and kafka sinks that are "+
"enabled by changefeed.new__sink_enabled only): <0 disables, 0 assigns "+
"a reasonable default, >0 assigns the setting value",
```

It seems that we use positive values to assign the setting value and otherwise do the default, but do not disable on negative values.

Jira issue: CRDB-56188