Clean up null mask after purging null entries

[sperlingxx]

After puring null entries via copy_if, the null mask of the input column will be preversed, despite it is useless. We can clean up the unused null mask to save some device memory.

[codecov]

Codecov Report

Merging #10412 (57ecb1c) into branch-22.04 (b3dc9d6) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 57ecb1c differs from pull request most recent head 2a36168. Consider uploading reports for the commit 2a36168 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.04   #10412      +/-   ##
================================================
+ Coverage         86.12%   86.16%   +0.03%     
================================================
  Files               139      139              
  Lines             22450    22460      +10     
================================================
+ Hits              19336    19352      +16     
+ Misses             3114     3108       -6     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	88.39% <ø> (+0.10%)	⬆️
python/cudf/cudf/core/column/column.py	88.99% <100.00%> (-0.02%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.07% <100.00%> (-0.05%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.98% <100.00%> (+0.29%)	⬆️
python/cudf/cudf/core/dataframe.py	93.57% <100.00%> (+<0.01%)	⬆️
python/cudf/cudf/core/frame.py	91.72% <100.00%> (ø)
python/cudf/cudf/core/groupby/groupby.py	91.92% <100.00%> (+0.41%)	⬆️
python/cudf/cudf/core/index.py	92.19% <100.00%> (+<0.01%)	⬆️
python/cudf/cudf/core/indexed_frame.py	92.25% <100.00%> (ø)
python/cudf/cudf/core/multiindex.py	92.13% <100.00%> (ø)
... and 16 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 46ac622...2a36168. Read the comment docs.

[sperlingxx]

@gpucibot merge