Fix fused_map_reduce when result is a matrix

[blegat]

This changes the behavior because now the dot product of empty vectors is MA.Zero() :/
On the other hand, fused_map_reduce(::typeof(add_mul), ::Vector{Int}, ::Vector{Matrix{Int}}) cannot return anything else than MA.Zero() in case the vectors are empty.
This is now non-breaking. Note that fused_map_reduce(add_mul, a, b) gets called if the user does a' * b and if eltype(a) is a scalar and eltype(b) is an array then it's currently failing. This PR fixes this use case.

See also JuliaAlgebra/StarAlgebras.jl#40

Closes #318

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.67%. Comparing base (006efd4) to head (211701b).
Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   91.83%   91.67%   -0.16%     
==========================================
  Files          23       23              
  Lines        2205     2259      +54     
==========================================
+ Hits         2025     2071      +46     
- Misses        180      188       +8     

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[blegat]

This triggers a regression on Julia v1.6. We could start playing around adding type asserts like ::Union{Zero,T} but we could also just drop Julia v1.6

[odow]

Closing in favor of #320