Если строка пуста, то необходимо удалить её и сохранить текстовый документ. Как это сделать?

with open('3.txt', 'r') as f:
    text = f.read()
with open('3.txt', 'w') as f:

import re
 
with open('in.txt', 'r') as f_in, open('out.txt', 'w') as f_out:
    f_out.write(''.join([i for i in f_in.readlines() if not re.fullmatch(r'\s+', i)]))

>>> "" # однозначно пустая
''
>>> "\n" # пустая?
'\n'
>>> " " # пустая?
' '
>>>
>>> "                  \n" # пустая?
'                  \n'
>>>

import re
 
with open('in.txt', 'r') as f:
    text = f.readlines()
with open('in.txt', 'w') as f:
    f.write(''.join([i for i in text if not re.fullmatch(r'\s+', i)

>>> line="""1
2
3
 
5"""
>>> open("test.txt","w").write(line)
8
>>> sum(1 for line in open("test.txt") if line=="\n")
1
>>> open("test.txt").readlines()
['1\n', '2\n', '3\n', '\n', '5']
>>>

>>> open("test2.txt","w").writelines([line for line in open("test.txt") if line!="\n"])
>>> print(open("test2.txt").read())
1
2
3
5
>>>

>>> lines = [line for line in open("test.txt") if line!="\n"]
>>> open("test.txt","w").writelines(lines)
>>> print(open("test.txt").read())
1
2
3
5
>>