Fix returned column type when extracting from an empty list column

[jlowe]

Fixes #8024

Fixes cudf::lists::extract_list_element to return an empty column of the list's child column type when the list column is empty.

[codecov]

Codecov Report

Merging #8025 (d244e64) into branch-0.20 (51336df) will decrease coverage by 0.34%.
The diff coverage is 88.67%.

❗ Current head d244e64 differs from pull request most recent head 11937d0. Consider uploading reports for the commit 11937d0 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #8025      +/-   ##
===============================================
- Coverage        82.88%   82.54%   -0.35%     
===============================================
  Files              103      103              
  Lines            17668    17351     -317     
===============================================
- Hits             14645    14323     -322     
- Misses            3023     3028       +5     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/io/orc.py	86.80% <ø> (-0.10%)	⬇️
python/cudf/cudf/utils/cudautils.py	55.04% <25.00%> (-2.72%)	⬇️
python/cudf/cudf/utils/dtypes.py	83.22% <50.00%> (-0.22%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.41% <70.00%> (-0.02%)	⬇️
python/cudf/cudf/core/column/column.py	88.48% <75.00%> (-0.17%)	⬇️
python/cudf/cudf/core/column/decimal.py	90.08% <78.26%> (-2.85%)	⬇️
python/dask_cudf/dask_cudf/backends.py	89.20% <80.00%> (-0.38%)	⬇️
python/cudf/cudf/core/dataframe.py	90.75% <83.33%> (-0.11%)	⬇️
python/cudf/cudf/utils/utils.py	89.94% <95.00%> (+0.43%)	⬆️
... and 60 more

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[kkraus14]

@gpucibot merge