Thinking about this a bit more, this seems sensible, at least for square empty matrices. For square matrices, we define the condition number as $\Vert A \Vert \cdot \Vert A^{-1} \Vert$. For an empty matrix, the norm should be zero (because it's the additive identity), and the inverse of a square empty matrix is itself (since $Ax = b$ has a unique solution: an empty $b$ is unique, and empty $x$ solves it).

Returning 0 is also what Matlab does, it seems.

Agree, restricting to square A is reasonable.

Would be good to have a @test_throws to ensure it throws for non-square empty matrices.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.89%. Comparing base (57e9a0d) to head (507cb3b).
Report is 1 commits behind head on master.

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##           master    #1169   +/-   ##
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  Coverage   91.89%   91.89%           
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  Files          34       34           
  Lines       15355    15358    +3     
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+ Hits        14110    14113    +3     
  Misses       1245     1245           

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