Remove hidden input when multi select is empty

When using a multi select with `joinValues`, the hidden input never gets removed.

eg:

```
<Select
  name="field-name"
  value={value}
  options={options}
  multi
  simpleValue
  delimiter="|"
  joinValues
/>
```

When the select is empty/cleared, the input still exists (with empty string as value):

```
<input type="hidden" name="field-name" value="">
```
