LinAlg: fzeropreserving unit triangular broadcast preserves structure (#53648)
On master
```julia
julia> UU = UnitUpperTriangular(reshape([1:9;],3,3))
3×3 UnitUpperTriangular{Int64, Matrix{Int64}}:
1 4 7
⋅ 1 8
⋅ ⋅ 1
julia> UU .* 2
3×3 Matrix{Int64}:
2 8 14
0 2 16
0 0 2
```
This PR
```julia
julia> UU .* 2
3×3 UpperTriangular{Int64, Matrix{Int64}}:
2 8 14
⋅ 2 16
⋅ ⋅ 2
```
This also improves performance, as the `materialize` skips the
structured zeros.
```julia
julia> UU = UnitUpperTriangular(rand(100, 100));
julia> @btime $UU .* 2;
12.788 μs (3 allocations: 78.20 KiB) # master
7.821 μs (3 allocations: 78.20 KiB) # PR
```
julia
1ba83f02 - LinAlg: fzeropreserving unit triangular broadcast preserves structure (#53648)
