BUG: String fields in compound dtypes don't resize as might be expected

Example:

```python
>>> dt = np.dtype([('f', float), ('s', str)])
>>> np.array([(1.0, "hello world")], dt)
array([(1.0, '')], 
      dtype=[('f', '<f8'), ('s', 'S')])
```

Proposal: This should either resize or raise a ValueError (#8970), as it seems like it would always mask bugs