Optimize array intersection functions for empty operands

[mehmetcansahin]

Avoid unnecessary iteration when an empty operand guarantees an empty result for array_intersect_key(), array_intersect_assoc(), and array_uintersect_assoc().

Includes regression tests covering validation and observable callback/warning behavior.

[kamil-tekiela]

How does it affect performance for the negative case when none of the arrays is empty?

[mehmetcansahin]

@kamil-tekiela Thanks for the review. I reran the non-empty cases using 15 paired processes per scenario and repeated the measurements twice.

For very small arrays, the overhead ranged from roughly 0.5% to 5%, corresponding to only a few nanoseconds per call. With 100 or 10,000 elements, the differences were generally around 1% or less and were not consistent between runs.

Overall, the shortcut adds a small fixed cost to non-empty calls, which becomes negligible as the arrays grow.