add nil expr for summarise and mutate - Py Polars lit(None)

[lkarthee]

This PR add lit(None). Can be used to align describe with Py Polars.

df = DF.new(strs: ["a", "b", "c"], nums: [1, 2, 3])
DF.mutate(df, c: nil)
#Explorer.DataFrame<
  Polars[3 x 3]
  strs string ["a", "b", "c"]
  nums s64 [1, 2, 3]
  c null [nil, nil, nil]
>
DF.summarise(df, c: nil)
#Explorer.DataFrame<
  Polars[1 x 1]
  c null [nil]
>

I will add test cases after review.

[josevalim]

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