improve type stability in jacobian for a certain case

[KristofferC]

See test. It seems this started failing in 1.7 for some reason:

julia> VERSION
v"1.7.0-rc3"

julia> using ForwardDiff

julia> g!(dy, y) = dy[1] = y[1]
g! (generic function with 1 method)

julia> @inferred ForwardDiff.jacobian(g!, [1.0], [0.0])
ERROR: return type Matrix{Float64} does not match inferred return type Any