Avoid copy in empty bidiag ldiv!

[jishnub]

If N == 0, the arrays are all empty, so there's nothing to copy. We may return the output array directly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.03%. Comparing base (5d3ef46) to head (02045d4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
+ Coverage   92.01%   92.03%   +0.01%     
==========================================
  Files          34       34              
  Lines       15488    15488              
==========================================
+ Hits        14251    14254       +3     
+ Misses       1237     1234       -3     

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