MG WCC bug fix

[seunghwak]

create_graph_edgelist (invoked by WCC to create a graph from the edges emitted when concurrently growing BFS trees touch each other) called renumber_edgelist with a valid vertex list argument when invoked by WCC with an invalid vertex list (std::nullopt). This fix checks the validity of the vertex list and calls renumber_edgelist with a valid vertex list only when this function is called with a valid vertex list.

[codecov-commenter]

Codecov Report

Merging #1802 (9d25030) into branch-21.10 (bf64c2c) will increase coverage by 12.27%.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           branch-21.10    #1802       +/-   ##
=================================================
+ Coverage         59.85%   72.13%   +12.27%     
=================================================
  Files                77      134       +57     
  Lines              3547     8311     +4764     
=================================================
+ Hits               2123     5995     +3872     
- Misses             1424     2316      +892     

Impacted Files	Coverage Δ
python/cugraph/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/_version.py	44.80% <ø> (ø)
python/cugraph/cugraph/centrality/__init__.py	100.00% <ø> (ø)
...graph/cugraph/centrality/betweenness_centrality.py	90.00% <ø> (ø)
...thon/cugraph/cugraph/centrality/katz_centrality.py	89.47% <ø> (ø)
python/cugraph/cugraph/comms/comms.py	34.52% <ø> (ø)
python/cugraph/cugraph/community/__init__.py	70.37% <ø> (ø)
python/cugraph/cugraph/community/ecg.py	100.00% <ø> (ø)
python/cugraph/cugraph/community/egonet.py	92.30% <ø> (ø)
...ython/cugraph/cugraph/community/ktruss_subgraph.py	84.37% <ø> (ø)
... and 183 more

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[ChuckHastings]

@gpucibot merge