All jax backend reducers give wrong length output if the last element of the input jagge arrays is empty

[ikrommyd]

All the reducers ignore the last element as we're not properly assigning the right num_segments and that causes them to ignore the last element which is empty

In [5]: import awkward as ak
   ...: ak.jax.register_and_check()
   ...:
   ...: x = ak.sum(ak.Array([[1, 2], [], [6,4,5], []], backend="cpu"), axis=1)
   ...: y = ak.sum(ak.Array([[1, 2], [], [6,4,5], []], backend="jax"), axis=1)

In [6]: x
Out[6]: <Array [3, 0, 15, 0] type='4 * int64'>

In [7]: y
Out[7]: <Array [3, 0, 15] type='3 * int64'>

Similar results are observed with every reducer.

[ikrommyd]

This is the more general issue that is seen in #3456 with the only difference that the other reducers are silently incorrect without erroring.