Some issues about processing empty strings into NaT when using pandas to_datetime
Code and results:

```python
import pandas as pd

data = [{'dt': '2020-01-11T11:00:00'},{'dt': ''}, {'dt': '2021-04-19T10:00:00+08'}]
df1 = pd.DataFrame(data)
df2 = pd.DataFrame(data)
assert df1['dt'].dtype.name == 'object'
assert df2['dt'].dtype.name == 'object'
df1['dt'] = pd.to_datetime(df1['dt'])
df2['dt'] = df2['dt'].apply(pd.to_datetime)
print(df1['dt'][1], type(df1['dt'][1]))   # Result: str: 'NaT'
print(df2['dt'][1], type(df2['dt'][1]))   # Result: pandas._libs.tslibs.nattype.NaTType: NaT
```