Preserve structure in scaling triangular matrices by NaN (#55310)
Addresses the `Matrix` cases from
https://github.com/JuliaLang/julia/issues/55296. This restores the
behavior to match that on v1.10, and preserves the structure of the
matrix on scaling by `NaN`. This behavior is consistent with the
strong-zero behavior for other structured matrix types, and the scaling
may be seen to be occurring in the vector space corresponding to the
filled elements.
After this,
```julia
julia> UpperTriangular(rand(2,2)) * NaN
2×2 UpperTriangular{Float64, Matrix{Float64}}:
NaN NaN
⋅ NaN
```
cc. @mikmoore
