feat(python): Add default to_struct Series name consistent with the usual default Series name (empty string)

[alexander-beedie]

Closes #12947.

Seems reasonable to be consistent here; the standard behaviour when creating a Series directly (without giving it an explicit name) is to assign the default name as the empty string.

Example

The following now behaves consistently:

import polars as pl
records = [ {'a':1,'b':2}, {'a':2,'b':4} ]

pl.Series( values=records )
pl.DataFrame( records ).to_struct()

# shape: (2,)
# Series: '' [struct[2]]
# [
#   {1,2}
#   {2,4}
# ]

Previously the second of these would result in the following error:

TypeError: 
  DataFrame.to_struct() missing 1 required positional argument: 'name'

[stinodego]

Yep seems reasonable, thanks!