Fix Cython compilation warnings.

[bdice]

This PR fixes three small issues relating to uninitialized variables in Cython code. This removes warnings from the build log and fixes potential bugs.

• Parquet code had a boolean variable is_range_index that was not initialized if json_str != "".
• Reduction scan type wasn't correctly initialized (if True: / elif False: pattern)
• Undesired comparison of a signed int and an unsigned value in a utility function.

[bdice]

rerun tests

[bdice]

rerun tests

[codecov]

Codecov Report

Merging #9327 (41793eb) into branch-21.12 (ab4bfaa) will increase coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head 41793eb differs from pull request most recent head 4f9edfa. Consider uploading reports for the commit 4f9edfa to get more accurate results

@@               Coverage Diff                @@
##           branch-21.12    #9327      +/-   ##
================================================
+ Coverage         10.79%   10.81%   +0.02%     
================================================
  Files               116      116              
  Lines             18869    18873       +4     
================================================
+ Hits               2036     2042       +6     
+ Misses            16833    16831       -2     

Impacted Files	Coverage Δ
python/cudf/cudf/core/_base_index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/dataframe.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/multiindex.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/series.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/backends.py	82.31% <0.00%> (+3.65%)	⬆️

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[shwina]

@gpucibot merge