Fix adjtransvec multiplication with AbstractZerosMatrix

[jishnub]

On master,

julia> [1,2]' * Zeros(2,2)
1×2 Zeros{Float64}

After this PR,

julia> [1,2]' * Zeros(2,2)
1×2 adjoint(::Zeros{Float64, 1, Tuple{Base.OneTo{Int64}}}) with eltype Float64:
  ⋅    ⋅ 

This matches the behavior of zeros

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (4c50186) to head (d6ce6e5).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #341   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           8        8           
  Lines        1043     1058   +15     
=======================================
+ Hits         1042     1057   +15     
  Misses          1        1           

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