Plotting nyquist plot from FrequencyResponseData fails

When trying to create the nyquist plot from an FRD system
```python
freq_response = ct.FRD(response, frequencies)
ct.nyquist_plot(freq_response)
```
the plotting fails with the following error
`AttributeError: 'FrequencyResponseData' object has no attribute 'poles'` during the call of `_default_frequency_range`

version: 0.10.0