Fix -, conj, and conj! for sparse matrices with invalid entries in nzval (#31187)

(cherry picked from commit ca92c7b)

Author: martinholters

stdlib/SparseArrays/test/sparse.jl

@@ -2586,4 +2586,16 @@ end
     @test sum(x1, dims=2) == sum(x2, dims=2)
 end
 
+@testset "unary operations on matrices where length(nzval)>nnz" begin
+    # this should create a sparse matrix with length(nzval)>nnz
+    A = SparseMatrixCSC(Complex{BigInt}[1+im 2+2im]')'[1:1, 2:2]
+    # ...ensure it does! If necessary, the test needs to be updated to use
+    # another mechanism to create a suitable A.
+    @assert length(A.nzval) > nnz(A)
+    @test -A == fill(-2-2im, 1, 1)
+    @test conj(A) == fill(2-2im, 1, 1)
+    conj!(A)
+    @test A == fill(2-2im, 1, 1)
+end
+
 end # module