DatetimeIndex.to_period with freq
- [X] I have checked that this issue has not already been reported.
- [X] I have confirmed this bug exists on the latest version of pandas (1.0.3).
---

#### Code Sample

```python
# Input date range:
pi5m = pd.date_range(start='2019-12-22 06:40:00+00:00', end='2019-12-22 08:45:00+00:00', freq='5min')

# Converting to PeriodIndex
pi5m.to_period()

  File "/home/p/.local/lib/python3.7/site-packages/pandas/core/arrays/period.py", line 938, in dt64arr_to_periodarr
    freq = Period._maybe_convert_freq(freq)

  File "pandas/_libs/tslibs/period.pyx", line 1580, in pandas._libs.tslibs.period._Period._maybe_convert_freq

AttributeError: 'NoneType' object has no attribute 'n'

# Working
pi5m.to_period('5min')

```

#### Problem description

It is specified in documentation that if not provided, `freq` is inferred.
However, with above-provided input, I get the error message indicated.
Is this a bug?

Thanks for your feedback.
Bests,