Matrix conversion for structured matrices without a zero

[jishnub]

This adds a fallback branch in case the eltype doesn't have a unique zero, so that the following works after this PR:

julia> D = Diagonal(fill(ones(2,2), 2, 2))
2×2 Diagonal{Matrix{Float64}, Vector{Matrix{Float64}}}:
 [1.0 1.0; 1.0 1.0]          ⋅         
         ⋅           [1.0 1.0; 1.0 1.0]

julia> Matrix(D)
2×2 Matrix{Matrix{Float64}}:
 [1.0 1.0; 1.0 1.0]  [0.0 0.0; 0.0 0.0]
 [0.0 0.0; 0.0 0.0]  [1.0 1.0; 1.0 1.0]

Similarly, for Bidiagonal and Tridiagonal/SymTridiagonal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.84%. Comparing base (6e5ea12) to head (d05a39b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1159      +/-   ##
==========================================
+ Coverage   91.83%   91.84%   +0.01%     
==========================================
  Files          34       34              
  Lines       15363    15375      +12     
==========================================
+ Hits        14108    14121      +13     
+ Misses       1255     1254       -1     

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