Fix multiplication with empty HessenbergQ (#1326)

(cherry picked from commit b14390e)

Author: dkarrasch

src/lapack.jl

@@ -6130,10 +6130,11 @@ for (ormhr, elty) in
             chkside(side)
             chktrans(trans)
             n = checksquare(A)
+            ntau = length(tau)
             mC, nC = size(C, 1), size(C, 2)
 
-            if n - length(tau) != 1
-                throw(DimensionMismatch(lazy"tau has length $(length(tau)), needs $(n - 1)"))
+            if max(n - 1, 0) != ntau
+                throw(DimensionMismatch(lazy"tau has length $ntau, needs $(n - 1)"))
             end
             if (side == 'L' && mC != n) || (side == 'R' && nC != n)
                 throw(DimensionMismatch("A and C matrices are not conformable"))

test/hessenberg.jl

@@ -286,4 +286,11 @@ end
     end
 end
 
+@testset "multiplication with empty HessenbergQ" begin
+    @test ones(2, 0)*hessenberg(zeros(0,0)).Q == zeros(2,0)
+    @test_throws DimensionMismatch ones(2, 1)*hessenberg(zeros(0,0)).Q
+    @test hessenberg(zeros(0,0)).Q * ones(0, 2) == zeros(0,2)
+    @test_throws DimensionMismatch hessenberg(zeros(0,0)).Q * ones(1, 2)
+end
+
 end # module TestHessenberg