[Python] Check if all values of Polars DataFrame are True

How can I check if all values of a polars DataFrame, containing only boolean columns, are True?
Example df:

df = pl.DataFrame({"a": [True, True, None],
"b": [True, True, True],
})


The reason for my question is that sometimes I want to check if all values of a df fulfill a condition, like in the following:

df = pl.DataFrame({"a": [1, 2, None],
"b": [4, 5, 6],
}).select(pl.all() >= 1)


By the way, I didn't expect that .select(pl.all() >= 1) keeps the null (None) in last row of column "a", maybe that's worth noting.

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