Fix TypeError with empty axes in FFT functions

[antonwolfy]

This PR fixes "copyto() argument 1 must be a numpy.ndarray" error, like:

deploy/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_place/lib/python3.13/site-packages/mkl_fft/_fft_utils.py", line 250, in _iter_complementary
    np.copyto(result[tuple(sl)], func(x[tuple(sl)], **kwargs))
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: copyto() argument 1 must be a numpy.ndarray, not numpy.complex128

when calling FFT functions with axes=().

Thus the PR adds a fast path to handle empty axes as identity transform, matching NumPy behavior.

The issue was introduced in scope of #283. The tests coverage is improved.

[jharlow-intel]

linux validation failure is due to bdba server outage, nothing to do with this PR.

LGTM, thanks for adding the new test cases, too

[ndgrigorian]

other than previous comment LGTM