Prevent saving null log lines to the database

[elias-ba]

Notes for the reviewer

Related issue

Fixes #866

Review checklist

• I have performed a self-review of my code
• I have verified that all appropriate authorization policies have been implemented and tested
• If needed, I have updated the changelog
• Taylor has QA'd this feature

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (0bcc598) 92.01% compared to head (563fe07) 92.04%.

❗ Current head 563fe07 differs from pull request most recent head 08856f1. Consider uploading reports for the commit 08856f1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #868      +/-   ##
==========================================
+ Coverage   92.01%   92.04%   +0.02%     
==========================================
  Files         164      164              
  Lines        4271     4274       +3     
==========================================
+ Hits         3930     3934       +4     
+ Misses        341      340       -1     

Impacted Files	Coverage Δ
lib/lightning/invocation.ex	97.56% <100.00%> (+0.03%)	⬆️
lib/lightning/invocation/log_line.ex	100.00% <100.00%> (ø)
lib/lightning/pipeline/runner.ex	97.95% <100.00%> (ø)
lib/lightning_web/live/run_live/components.ex	98.61% <100.00%> (+0.69%)	⬆️

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[taylordowns2000]

please see my one in-line question. otherwise, pending code review from @stuartc and full click testing from @amberrignell , this looks good to go

[stuartc]

👍 Nice.