add undefmatrix for numbers

Author: oscardssmith

lib/ArrayInterfaceCore/src/ArrayInterfaceCore.jl

@@ -526,7 +526,9 @@ returns the matrix linear operator type which acts on the `vec` of the array.
 function undefmatrix(u)
     similar(u, length(u), length(u))
 end
-                                                                                                                                    
+function undefmatrix(u::Number)
+    return zero(u)
+end
 """
     restructure(x,y)
 

lib/ArrayInterfaceCore/test/runtests.jl

@@ -25,6 +25,8 @@ Aqua.test_all(ArrayInterfaceCore)
         @test size(um) == (length(v),length(v))
         @test typeof(um) == Matrix{T}
         @test zeromatrix(v) == zeros(T,4*4*4,4*4*4)
+        @test zeromatrix(rand(T)) == zero(T)
+        @test undefmatrix(rand(T)) isa T
     end
 end