Skip empty sublists with no actual invalidations

[timholy]

Fixes #354

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (05d2237) 85.85% compared to head (70863c1) 85.86%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #355   +/-   ##
=======================================
  Coverage   85.85%   85.86%           
=======================================
  Files          17       17           
  Lines        2192     2193    +1     
=======================================
+ Hits         1882     1883    +1     
  Misses        310      310           

Impacted Files	Coverage Δ
src/invalidations.jl	72.97% <100.00%> (+0.05%)	⬆️

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