df.date_of_last_hoorah_given = pd.to_datetime(df.date_of_last_hoorah_given)
df = df.sort_values(by='date_of_last_hoorah_given')
mask = df.date_of_last_hoorah_given.isnull()
print(pd.concat([df[mask], df[~mask]]))
sort Pandas dataframe with NaT values on top