fix: explicit primitive promotion for scalar typetracer arrays

[pfackeldey]

This fixes: #578 (comment)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.10%. Comparing base (8cb8994) to head (91e1d64).
Report is 254 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #580      +/-   ##
==========================================
- Coverage   93.06%   92.10%   -0.97%     
==========================================
  Files          23       24       +1     
  Lines        3290     3596     +306     
==========================================
+ Hits         3062     3312     +250     
- Misses        228      284      +56     

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[martindurant]

OK, I guess. I can't see any obvious down sides of this. Can we check that no copies are made in the normal case? Is it just as fast?

[pfackeldey]

This is just restoring the original behavior, there shouldn't be any performance drawbacks. ak.Array would internally use ak.to_layout to promote a 0-dim typetracer array before. Now we do that explicitly and ak.Array skips this step then.

[martindurant]

OK +1