bug: cache probe still performed even if count == 0

A common practice I've found for the `envbuilder_cached_image` resource is the following:

```
resource "envbuilder_cached_image" "cached" {
  count = data.coder_workspace.me.start_count
  ...
}
```

However, when `start_count` is `0` I've still observed the resource being re-created.
This results in workspace shutdowns taking longer than they need, especially for large images.