Make dot consistently return zero on empty arrays

[dkarrasch]

Fixes #1483. I checked all 2-arg dot methods and found two to be fixed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.94%. Comparing base (fa00b3c) to head (0bc71d2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1494      +/-   ##
==========================================
+ Coverage   93.92%   93.94%   +0.02%     
==========================================
  Files          34       34              
  Lines       15979    15980       +1     
==========================================
+ Hits        15008    15013       +5     
+ Misses        971      967       -4     

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