对分组的pandas数据帧中的行求和并返回NaN

import pandas as pd

# 加载数据帧
df = pd.DataFrame({'group': ['A', 'A', 'B', 'B', 'C'],
                   'col1': [1, 2, 3, 4, 5],
                   'col2': [6, 7, 8, 9, 10]})

def sum_with_nan(group):
    if len(group) == 0:
        return pd.Series({'col1': pd.NaT, 'col2': pd.NaT})
    else:
        return group[['col1', 'col2']].sum()

# 对分组的行求和并返回NaN
result = df.groupby('group').apply(sum_with_nan)