src: stop package search at empty package.json

[Archkon]

Represent package contents with std::optional so a successfully read empty file is distinct from a missing file. This prevents node --run from falling back to scripts in a parent package.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (73236f3) to head (b3b89cf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64918   +/-   ##
=======================================
  Coverage   90.31%   90.31%           
=======================================
  Files         759      759           
  Lines      248295   248298    +3     
  Branches    46863    46863           
=======================================
+ Hits       224242   224245    +3     
- Misses      15460    15467    +7     
+ Partials     8593     8586    -7     

Files with missing lines	Coverage Δ
src/node_task_runner.cc	91.71% <100.00%> (+2.38%)	⬆️

... and 30 files with indirect coverage changes

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