Specialize haszero for Union{Missing,<:Number}

[jishnub]

Since zero(::Union{Missing,T}) calls zero(T) internally, we may use the same logic for LinearAlgebra.haszero. This helps with certain structured matrix operations:

julia> M = Matrix{Union{Int,Missing}}(missing,2,2)
2×2 Matrix{Union{Missing, Int64}}:
 missing  missing
 missing  missing

julia> triu(M)
2×2 Matrix{Union{Missing, Int64}}:
  missing  missing
 0         missing

whereas previously, this would have been

julia> triu(M)
2×2 Matrix{Union{Missing, Int64}}:
 missing  missing
 missing  missing