guard against empty shape arrays by setting initial value in reduce operations over state dimensions in State

[tclose]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

Fixes #757 by providing initial value to reduce functions

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.75%. Comparing base (55c30c1) to head (7fccfef).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #818   +/-   ##
=======================================
  Coverage   88.75%   88.75%           
=======================================
  Files          84       84           
  Lines       17891    17891           
  Branches     3487     3483    -4     
=======================================
  Hits        15879    15879           
  Misses       1642     1642           
  Partials      370      370           

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[effigies]

I think it's clearer just to use math.prod (needs import), rather than reimplement it with reduce.