Fix abort in top_k for k=0

[antonwolfy]

dpnp.tensor.top_k(x, 0) might aborted the process on an assertions-enabled SYCL runtime:

import dpnp.tensor as dpt
dpt.top_k(dpt.arange(12).reshape(3, 4), 0)   # k = 0

k == 0 is a valid request whose result is empty, and it passes validation (k < 0 is rejected, k > size is rejected, but k == 0 is allowed). py_topk then dispatched to the kernel, whose output-writing step (write_out_impl) builds an nd_range from nelems = iter_nelems * k. A zero-sized nd_range is a silent no-op on runtimes built with NDEBUG, but aborts on an assertions-enabled SYCL runtime.

The existing guard covered iter_nelems == 0 and axis_nelems == 0, but not k == 0.

This PR proposes to extend the early-return guard in py_topk to also cover k == 0.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/3022/index.html

[coveralls]

coverage: 78.402% (-0.01%) from 78.412% — fix-empty-topk into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev3=py314h509198e_41 ran successfully.
Passed: 1376
Failed: 1
Skipped: 5