Destination zeros from eltype in zero-length generic_matvecmul (#1241)

This fixes issues like
```julia
julia> A = fill(@smatrix(zeros(2,2)), 4, 0)
4×0 Matrix{SMatrix{2, 2, Float64, 4}}

julia> v = fill(@smatrix(zeros(2,2)), size(A,2))
SMatrix{2, 2, Float64, 4}[]

julia> A * v
ERROR: MethodError: Cannot `convert` an object of type Bool to an object of type SMatrix{2, 2, Float64, 4}
The function `convert` exists, but no method is defined for this combination of argument types.
[...]
```

Author: jishnub

src/matmul.jl

@@ -932,7 +932,7 @@ function __generic_matvecmul!(f::F, C::AbstractVector, A::AbstractVecOrMat, B::A
     @inbounds begin
         if length(B) == 0
             for k = eachindex(C)
-                @stable_muladdmul _modify!(MulAddMul(alpha,beta), false, C, k)
+                @stable_muladdmul _modify!(MulAddMul(alpha,beta), zero(eltype(C)), C, k)
             end
         else
             for k = eachindex(C)
@@ -955,7 +955,7 @@ function __generic_matvecmul!(::typeof(identity), C::AbstractVector, A::Abstract
             if !iszero(beta)
                 C[i] *= beta
             elseif length(B) == 0
-                C[i] = false
+                C[i] = zero(eltype(C))
             else
                 C[i] = zero(A[i]*B[1] + A[i]*B[1])
             end

test/matmul.jl

@@ -6,6 +6,11 @@ using Base: rtoldefault
 using Test, LinearAlgebra, Random
 using LinearAlgebra: mul!, Symmetric, Hermitian
 
+const BASE_TEST_PATH = joinpath(Sys.BINDIR, "..", "share", "julia", "test")
+
+isdefined(Main, :SizedArrays) || @eval Main include(joinpath($(BASE_TEST_PATH), "testhelpers", "SizedArrays.jl"))
+using .Main.SizedArrays
+
 ## Test Julia fallbacks to BLAS routines
 
 mul_wrappers = [
@@ -1176,4 +1181,16 @@ end
     end
 end
 
+@testset "zero-length generic matvec" begin
+    m = SizedArrays.SizedArray{(2,2)}(ones(2,2))
+    A = fill(m, 2, 0)
+    v = fill(m, size(A,2))
+    w = similar(v, size(A,1))
+    mul!(w, A, v)
+    @test all(iszero, w)
+    A = fill(m, 0, 2)
+    mul!(w, A', v)
+    @test all(iszero, w)
+end
+
 end # module TestMatmul