fix: Scalar typetracer after reduction

[martindurant]

Fixes #528

This appears to fix the linked issue, but I'm not really sure why a typetracer object is being passed around rather than a high-level Array.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (8cb8994) to head (1f4e79d). Report is 116 commits behind head on main.

Files	Patch %	Lines
src/dask_awkward/lib/core.py	88.88%	1 Missing :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   93.06%   92.91%   -0.16%     
==========================================
  Files          23       23              
  Lines        3290     3389      +99     
==========================================
+ Hits         3062     3149      +87     
- Misses        228      240      +12     

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