AD norm is NaN at origin

Not really specific to `Tensors.jl`, but maybe it's good to document it here as well. I ran into the following issue yesterday
```
julia> using Tensors

julia> Tensors.hessian(x->norm(x),zero(Tensor{2,2}),:all)
([NaN NaN; NaN NaN;;; NaN NaN; NaN NaN;;;; NaN NaN; NaN NaN;;; NaN NaN; NaN NaN], [NaN NaN; NaN NaN], 0.0)
```

Basically a duplicate of https://github.com/JuliaDiff/ChainRules.jl/issues/576 and https://github.com/JuliaDiff/ForwardDiff.jl/issues/547 just at a different repo. You can close it if you think its too redundant.