GraphStore: handle zero-sized edge indices

[mananshah99]

Small fix for edge indices of shape (2, 0) (decomposed into two tensors of size 0), for which we cannot index into each tensor.

[codecov]

Codecov Report

Merging #4962 (fb16068) into master (6f62e53) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

❗ Current head fb16068 differs from pull request most recent head 6a70ab6. Consider uploading reports for the commit 6a70ab6 to get more accurate results

@@            Coverage Diff             @@
##           master    #4962      +/-   ##
==========================================
- Coverage   82.73%   82.73%   -0.01%     
==========================================
  Files         330      330              
  Lines       17901    17905       +4     
==========================================
+ Hits        14810    14813       +3     
- Misses       3091     3092       +1     

Impacted Files	Coverage Δ
torch_geometric/data/graph_store.py	92.54% <75.00%> (-0.45%)	⬇️

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[rusty1s]

Thank you!