Fix ValueError in dpnp.bincount for empty input arrays

[antonwolfy]

dpnp.bincount raised a ValueError when given an empty input array, while NumPy returns an empty (or zero-filled) array. This PR short-circuits the empty-input case so dpnp.bincount matches NumPy.

The Python layer determines the binning edges by reducing over the input with dpnp.max/dpnp.min. Those reductions have no identity for zero-size input, so an empty array failed before ever reaching the kernel:

>>> import dpnp
>>> dpnp.bincount(dpnp.array([], dtype="i8"))
ValueError: reduction cannot be performed over zero-size axes

The C++ kernel already guards sample.get_size() == 0, but that code path is unreachable because the max/min reduction runs first.

The fix assumes to return early for empty input with an intp array of zeros of length minlength, before any reduction is performed.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev3=py314h509198e_40 ran successfully.
Passed: 1376
Failed: 1
Skipped: 5

[coveralls]

coverage: 78.402% (-0.01%) from 78.414% — fix/bincount-empty-input into master