Skip empty sublists with no actual invalidations

[timholy]

Fixes #354

[codecov[bot]]

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

```diff @@ 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](https://codecov.io/gh/timholy/SnoopCompile.jl/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tim+Holy) | Coverage Δ | | |---|---|---| | [src/invalidations.jl](https://codecov.io/gh/timholy/SnoopCompile.jl/pull/355?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tim+Holy#diff-c3JjL2ludmFsaWRhdGlvbnMuamw=) | `72.97% <100.00%> (+0.05%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tim+Holy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tim+Holy)

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