Fix JAX 0-d array spacing crash in make_axis

[SGavrl]

The first snippet in the GPU / JAX Support section of the README crashes:

  x  = jnp.linspace(0, 2 * jnp.pi, 1000)
  dx = x[1] - x[0]
  Diff(0, dx)(jnp.sin(x))    # TypeError: len() of unsized object

dx is a JAX 0-d array, which isn't a numbers.Number, so make_axis routes
it to the non-uniform path and len(coords) fails. Same crash applies to numpy
and CuPy 0-d arrays.

Fix: in the is_array branch, treat ndim == 0 as scalar spacing. Regression
test added under TestJAXBackend.