Allow Series.fill_missing/2 to receive infinity and neg_infinity values

[Jhonatannunessilva]

This PR allows the Series.fill_missing/2 function to receive :infinity and :neg_infinity values.

This is part of #467

Before this PR, if you try to send a :infinity or :neg_infinity value to the Series.fill_missing/2 function, an error will be returned:

iex(1)> s1 = Explorer.Series.from_list([1.0, 2.0, nil, 4.5])            
#Explorer.Series<
  Polars[4]
  float [1.0, 2.0, nil, 4.5]
>

iex(2)> Explorer.Series.fill_missing(s1, :infinity)
** (ArgumentError) cannot invoke Explorer.Series.fill_missing/2 with mismatched dtypes: :float and :infinity
    (explorer 0.6.0-dev) lib/explorer/series.ex:3313: Explorer.Series.dtype_mismatch_error/3

iex(2)> Explorer.Series.fill_missing(s1, :neg_infinity)
** (ArgumentError) cannot invoke Explorer.Series.fill_missing/2 with mismatched dtypes: :float and :neg_infinity
    (explorer 0.6.0-dev) lib/explorer/series.ex:3313: Explorer.Series.dtype_mismatch_error/3

After this PR the result is:

iex(2)> Explorer.Series.fill_missing(s1, :infinity)         
#Explorer.Series<
  Polars[4]
  float [1.0, 2.0, Inf, 4.5]
>

iex(3)> Explorer.Series.fill_missing(s1, :neg_infinity)     
#Explorer.Series<
  Polars[4]
  float [1.0, 2.0, -Inf, 4.5]
>

[philss]

[josevalim]

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