Fix diagonal() strides calculation for empty results (#2814)

Previously `dpnp.diagonal()` returned incorrect strides when the
diagonal was empty (e.g., when offset >= array width). The stride was
set to itemsize instead of the correct `st_n + st_m`.

This PR:
* fixes stride calculation to consistently use (st_n + st_m)
* simplifies implementation using unified formula to calculate resulting
shape, strides, and offset
* extends tests coverage including scenarios with empty diagonals,
views, and non-contiguous arrays

The PR closes #2761. db486b9

Author: github-actions[bot]

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