fix: from_rows

[universalmind303]

closes #20

note: the explicit schema is required if you want to retain the null values.

// this will drop the null column if all values of "c" are null
> pl.DataFrame([{a:1, b:2, c:null}])

shape: (1, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ f64 ┆ f64 │
╞═════╪═════╡
│ 1.0 ┆ 2.0 │
└─────┴─────┘

// this will strictly use the schema provided.
> pl.DataFrame([{a:1, b:2, c:null}], {
  schema: { a: pl.Int32, b: pl.Int32, c: pl.Utf8 },
  orient: "row",
});

shape: (1, 3)
┌─────┬─────┬──────┐
│ a   ┆ b   ┆ c    │
│ --- ┆ --- ┆ ---  │
│ i32 ┆ i32 ┆ str  │
╞═════╪═════╪══════╡
│ 1   ┆ 2   ┆ null │
└─────┴─────┴──────┘