Copy matrices in triu/tril if no zero exists for the eltype (#1320)

This fixes a regression in the `triu`/`tril` implementation on v1.12 and
nightly. After this, the following works again:
```julia
julia> M = fill(ones(2,2), 3, 3)
3×3 Matrix{Matrix{Float64}}:
 [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]
 [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]
 [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]

julia> using Test: GenericArray

julia> triu(GenericArray(M),1)
3×3 GenericArray{Matrix{Float64}, 2}:
 [0.0 0.0; 0.0 0.0]  [1.0 1.0; 1.0 1.0]  [1.0 1.0; 1.0 1.0]
 [0.0 0.0; 0.0 0.0]  [0.0 0.0; 0.0 0.0]  [1.0 1.0; 1.0 1.0]
 [0.0 0.0; 0.0 0.0]  [0.0 0.0; 0.0 0.0]  [0.0 0.0; 0.0 0.0]
```

(cherry picked from commit c33045c)

Author: jishnub

test/dense.jl

@@ -1384,4 +1384,14 @@ end
     @test tril(GenericArray(M),-1) == res'
 end
 
+@testset "triu/tril for block matrices" begin
+    O = ones(2,2)
+    Z = zero(O)
+    M = fill(O, 3, 3)
+    res = fill(Z, size(M))
+    res[1,2] = res[1,3] = res[2,3] = O
+    @test triu(GenericArray(M),1) == res
+    @test tril(GenericArray(M),-1) == res'
+end
+
 end # module TestDense