Fix init of DataFrame with empty dataset (eg:"[]") and column/schema typedefs

[alexander-beedie]

Fixes #3321; when input data is an empty sequence, construct the DataFrame using the same codepath as in the None case. Extended current test coverage to validate.

Example:

import polars as pl
pl.DataFrame( data=[], columns=[("col1",pl.Float64),("col2",pl.Float64)] )

Without fix (error):

RuntimeError: Duplicate("Column with name: '' has more than one occurrences")

With fix (empty dataframe with correct schema):

shape: (0, 2)
┌──────┬──────┐
│ col1 ┆ col2 │
│ ---  ┆ ---  │
│ f64  ┆ f64  │
╞══════╪══════╡
└──────┴──────┘

[ritchie46]

Thanks!