rand_tangent and zero-arrays.

[AlexRobson]

Bit of an edge case but:

using Test
using FiniteDifferences

x = rand(2,2)
@test typeof(rand_tangent(x)) == typeof(x) # Passes
x = rand(1,)
@test typeof(rand_tangent(x)) == typeof(x) # Passes
x = fill(rand())
@test typeof(rand_tangent(x)) == typeof(x) # Fails

idk how often this will come up, but in the case that x is a zero-array, presently rand_tangent will change it from Array{T, 0} to T. It came up when testing some test_rrules with zero-arrays.

I think defining rand_tangent(rng::AbstractRNG, x::StridedArray{T, 0}) where {T} would be sufficient here? So simple to fix, if it's wanted.