fix: length 0 slices

[pfackeldey]

This fixes slices like array[0:0]. Without this PR dask-awkward would wrongly slice nothing at all, ie returning the original unsliced array.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.10%. Comparing base (8cb8994) to head (c9815f1).
Report is 256 commits behind head on main.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #581      +/-   ##
==========================================
- Coverage   93.06%   92.10%   -0.97%     
==========================================
  Files          23       24       +1     
  Lines        3290     3596     +306     
==========================================
+ Hits         3062     3312     +250     
- Misses        228      284      +56     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.