Multiplication between `ScalMat` and `AbstractZeros` is ambiguous.

[Wu-Chenyang]

The following code runs into errors.

julia> using PDMats, FillArrays
julia> ScalMat(3, 1) * Zeros(3)
ERROR: MethodError: *(::ScalMat{Int64}, ::Zeros{Float64, 1, Tuple{Base.OneTo{Int64}}}) is ambiguous.

Candidates:
  *(a::ScalMat, x::AbstractVector)
    @ PDMats ~/.julia/packages/PDMats/cAM9h/src/scalmat.jl:43
  *(a::AbstractMatrix{T}, b::FillArrays.AbstractZeros{T, 1} where T) where T
    @ FillArrays ~/.julia/packages/FillArrays/lVl4c/src/fillalgebra.jl:103

Possible fix, define
  *(::ScalMat{T}, ::FillArrays.AbstractZeros{T, 1} where T) where T<:Real

Similarly, the matrix multiplication also fails.

julia> ScalMat(3,1) * Zeros(3,3)
ERROR: MethodError: *(::ScalMat{Int64}, ::Zeros{Float64, 2, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}) is ambiguous.

Candidates:
  *(a::ScalMat, x::AbstractMatrix)
    @ PDMats ~/.julia/packages/PDMats/cAM9h/src/scalmat.jl:47
  *(a::AbstractMatrix, b::AbstractZeros{T, 2} where T)
    @ FillArrays ~/.julia/packages/FillArrays/lVl4c/src/fillalgebra.jl:98

Possible fix, define
  *(::ScalMat, ::AbstractZeros{T, 2} where T)

Higher dimensional multiplication wouldn't run into this problem because there isn't a method between ScalMat and AbstractArray. Therefore, an easy fix is, as suggested, implementing two extra methods in FillArraysPDMatsExt.jl. Appending the following code to the end of FillArraysPDMatsExt.jl should work.

using FillArrays: mult_zeros, AbstractZeros
Base.:*(a::ScalMat, b::AbstractZeros{T, 1} where T) = mult_zeros(a, b)
Base.:*(a::ScalMat, b::AbstractZeros{T, 2} where T) = mult_zeros(a, b)

[jishnub]

A PR is welcome

[jishnub]

Closed by #403