[12.x] Improved type hints for when() helper function.

[marcreichel]

This pull request adds more detailed type hints to the when() helper function, allowing PHPStan to better analyze the output. Allowing the usage of when() in higher PHPStan levels (9+) where the output of when() gets provided to another method that requires a specific parameter type.

Examples

// Before
when(random_int(0, 1), 42); // <- mixed

// After
when(random_int(0, 1), 42); // <- 42|null

// Before
when(true, 42); // <- mixed

// After
when(true, 42); // <- 42

// Before
when(random_int(0, 1), 42, 1337); // <- mixed

// After
when(random_int(0, 1), 42, 1337); // <- 42|1337

[github-actions]

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[shaedrich]

I could have sworn, these types were already part of the helper—I couldn't have been more wrong apparently 🤯

All the better, that someone took on the task 👍🏻

[shaedrich]

LGTM 👍🏻

But I can neither approve nor reject your PR, however, Taylor will eventually.