Allow None outputs in FeatureStore

[rusty1s]

Moves any KeyError logic to _get_tensor()

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 89.58%. Comparing base (ceeea03) to head (3847f4b).

Files	Patch %	Lines
torch_geometric/data/feature_store.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9102   +/-   ##
=======================================
  Coverage   89.58%   89.58%           
=======================================
  Files         470      470           
  Lines       30160    30160           
=======================================
  Hits        27018    27018           
  Misses       3142     3142           

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