Destination zeros from eltype in zero-length generic_matvecmul

[jishnub]

This fixes issues like

julia> A = fill(@SMatrix(zeros(2,2)), 4, 0)
4×0 Matrix{SMatrix{2, 2, Float64, 4}}

julia> v = fill(@SMatrix(zeros(2,2)), size(A,2))
SMatrix{2, 2, Float64, 4}[]

julia> A * v
ERROR: MethodError: Cannot `convert` an object of type Bool to an object of type SMatrix{2, 2, Float64, 4}
The function `convert` exists, but no method is defined for this combination of argument types.
[...]

[jishnub]

@KristofferC The linux-i686 job is running out of time frequently, and is being terminated after 1 hr. Would it be possible to increase the time limit for the job?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.02%. Comparing base (7f1dfe4) to head (451f015).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1241       +/-   ##
===========================================
+ Coverage    0.00%   92.02%   +92.02%     
===========================================
  Files          34       34               
  Lines       15738    15461      -277     
===========================================
+ Hits            0    14228    +14228     
+ Misses      15738     1233    -14505     

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