Make cudf::round for fixed_point when scale = -decimal_places a no-op

[codereport]

@nartal1 found a small bug while working on: NVIDIA/spark-rapids#1244

Problem is that for fixed_point, when the column scale = -decimal_places, it should be a no-op. Fix is to make it a no-op.

[codecov]

Codecov Report

Merging #6975 (da7d13e) into branch-0.18 (89938fa) will increase coverage by 0.42%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.18    #6975      +/-   ##
===============================================
+ Coverage        81.59%   82.01%   +0.42%     
===============================================
  Files               96       96              
  Lines            15905    16267     +362     
===============================================
+ Hits             12978    13342     +364     
+ Misses            2927     2925       -2     

Impacted Files	Coverage Δ
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/applyutils.py	98.74% <0.00%> (+0.02%)	⬆️
python/cudf/cudf/core/join/join.py	92.44% <0.00%> (+0.03%)	⬆️
... and 35 more

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[harrism]

The title of this PR reads like an issue title. For the sake of the changelog, can you please rewrite it to reflect what the PR does?