Fix empty diagindices, add more tests

[mtfishman]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.43%. Comparing base (b54a799) to head (267515f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/diaginterface/diaginterface.jl	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   80.50%   83.43%   +2.92%     
==========================================
  Files          11       11              
  Lines         159      169      +10     
==========================================
+ Hits          128      141      +13     
+ Misses         31       28       -3     

Flag	Coverage Δ
docs	64.07% <40.00%> (-2.18%)	⬇️

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