Fix tree_min and tree_max to handle zero-size array leafs.

[carlosgmartin]

Edit tree_min and tree_max to correctly handle pytrees that contain leafs that are zero-size arrays. For example:

Before:

$ py -c "import optax, jax; print(optax.tree.min([3, jax.numpy.zeros(0)]))"
ValueError: zero-size array to reduction operation min which has no identity

After:

$ py -c "import optax, jax; print(optax.tree.min([3, jax.numpy.zeros(0)]))"
3.0