why i have the output of [None, None, None]

[length power]

>>> x=['','x1','x2','x3','   ']
>>> x
['', 'x1', 'x2', 'x3', '   ']
>>> [print("ok") for it in x if it.strip() !=""]
ok
ok
ok
[None, None, None]

i understand there are three 'ok' in the output,but why i have the output
of [None, None, None]
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