feat: Do not panic when casting from an empty Series to pl.Decimal

[petrosbar]

Closes #14002

More specifically, the PR attempts to address the following two issues:

1. Panic when casting from an empty Series without explicitly setting a scale:

pl.Series().cast(pl.Decimal)
# Panic: no scale!

In this case, we just set scale to 0, in the same way as it's done when constructing an empty Series with dtype=pl.Decimal:

pl.Series([], dtype=pl.Decimal) 

# shape: (0,)
# Series: '' [decimal[*,0]]
# [
# ] 

2. An error is raised when casting a Series, whose dtype is already pl.Decimal, without setting scale explicitly:

from decimal import Decimal

pl.Series([Decimal("10.0")]).cast(pl.Decimal)
# ComputeError: no scale!

I don't know why anyone would want to do that but raising an error seems too punishing; the existing scale can be kept. For example:

from decimal import Decimal

pl.Series([Decimal("10.0")]).cast(pl.Decimal)

# shape: (1,)
# Series: '' [decimal[*,1]]
# [
#         10
# ]

[ritchie46]

Yeap, seems good to me. Thanks.