[9.x] Add whenMissing() method to InteractsWithInput trait

[phh]

The InteractsWithInput methods has() and filled() both have a syntactic sugar method whenHas() and whenFilled() that accepts a callback to avoid adding if's.

This PR adds the method whenMissing() that is basically the opposite of whenHas().

Today I was working on a middleware that takes care of both mappings and default fields. That looked something like this:

$request->mergeWhenMissing([
    'type' => 'default',
]);

However, sometimes you need to add a little more than just the additional fields. Could be something like notifying a 3rd part service that this part of the API is still getting used:

if ($request->whenMissing('type') {
    $this->deprecated('type');

    $request->merge(['type' => 'default']);
}

I just think it's a nice little syntactic sugar.