如何在包含NaNs的DataFrame列中查找小写单词？

import pandas as pd
import numpy as np
import re

data = {'col1': ['apple', 'Banana', np.nan, 'orange', 'grape'],
        'col2': ['cat', 'dog', 'elephant', np.nan, 'lion']}
df = pd.DataFrame(data)

def find_lowercase_words(text):
    lowercase_words = re.findall(r'\b[a-z]+\b', str(text).lower())
    return lowercase_words

df['lowercase_words'] = df['col1'].apply(find_lowercase_words)

print(df['lowercase_words'])