fix: Incorrect Decimal value for fill_null(strategy="one")

[lukemanley]

Fixes the following:

from decimal import Decimal
import polars as pl

ser = pl.Series([Decimal("1.23"), None])
ser.fill_null(strategy="one")

BEFORE:

shape: (2,)
Series: '' [decimal[*,2]]
[
        1.23
        0.01
]

AFTER:

shape: (2,)
Series: '' [decimal[*,2]]
[
        1.23
        1.00
]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.63%. Comparing base (9c1637c) to head (93bb046).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20844      +/-   ##
==========================================
- Coverage   79.63%   79.63%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      223146   223153       +7     
  Branches     2546     2546              
==========================================
- Hits       177711   177710       -1     
- Misses      44851    44859       +8     
  Partials      584      584              

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