array_change_key_case() should not mention return null on ZPP violation

[Sunthief]

From manual page: https://php.net/function.array-change-key-case

There seems to be an error in the function definition, more specifically the return value. This:
array_change_key_case(array $array, int $case = CASE_LOWER): array
needs to be altered to this:
array_change_key_case(array $array, int $case = CASE_LOWER): ?array
Null is returned if no array is passed.

[Girgias]

That's an issue with the return value section. As this is a ZPP violation, and as of PHP 8 always throws a TypeError

[dreeh]

#2849