fix: Incorrectly preserved sorted flag when concatenating sorted series containing nulls

[nameexhaustion]

Fixes #15072.

Existing releases are incorrectly setting the sorted flag:

s = pl.Series([None, 1])
out = pl.concat(3 * [s.sort()])
print(out)
shape: (4,)
Series: '' [i64]
[
        null
        1
        null
        1
]
print(out.flags)
{'SORTED_ASC': True, 'SORTED_DESC': False}
               ^^^^

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 81.03%. Comparing base (f08914b) to head (7f6c851).
Report is 5 commits behind head on main.

Files	Patch %	Lines
crates/polars-core/src/chunked_array/mod.rs	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15082      +/-   ##
==========================================
- Coverage   81.04%   81.03%   -0.02%     
==========================================
  Files        1338     1338              
  Lines      173771   173792      +21     
  Branches     2456     2456              
==========================================
- Hits       140832   140828       -4     
- Misses      32471    32496      +25     
  Partials      468      468              

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[ritchie46]

Thank you @nameexhaustion