[11.x] add lazy default to when helper

[rodrigopedra]

PR #52665 added a new helper to Laravel: when()

It simplifies the evaluation of a condition to output a value similar to PHP's ternary operator, with the false branch always returning null.

This PR:

• Augments the helper feature set by allowing a developer to define a default value in case the conditions evaluates to false.
• Aligns the implementation with the Illuminate\Support\Traits\Conditionable@when() method, by passing the $condition value to the value() helper, so it is passed down as an argument to any \Closure used as the true or default values.
• Adds new test assertions to account for the newly added parameter

Notes

Why not use the null-coalesce (??) operator?

One could argue that the $default parameter is not needed, as a developer could use the null-coalesce operator instead, like this:

{{ when($user, fn() => $user->preference('theme')) ?? 'light' }}

But that doesn't allow for the lazy evaluation of the $default value, in case we want to use a dynamic default value, for example, when using Pennant to test a new design:

{{ when(Auth::user(), fn($user) => $user->preference('theme'), fn() => Feature::active('site-redesign') ? 'new-theme' : 'old-theme') }}

In the example above, the $default value would be lazily evaluated when a guest is visiting the application.

Also, as we are passing the $condition value down to the truthy \Closure, we don't need to Auth::user again.

Breaking change

As a new parameter is added, it could be considered a breaking change.

But the when() helper was just added in today's release, also the $default parameter is optional with a null default value, preserving the original behavior.

Disclaimer

I suggested this implementation in the original PR in this comment: #52665 (comment)

But, although the author of that PR was very welcoming of all suggestions, the addition of the $default value didn't make the cut.

Post Script

I changed some of the original test assertions to use assertNull(...) instead of assertEquals(null, ...).

[devajmeireles]

I would use this PR to improve the description, I don't think the term "Output" makes sense in the description.

Also, I think $condition could have been named $boolean, like some other methods that deal with this kind of logic.

[shaedrich]

Why not just

return value($condition ? $value : $default, $condition);

?