Fix validator treating null as true for (required|exclude)_(if|unless) due to loose in_array() check

[jessarcher]

There's currently some unusual behavior with a number of validation rules that are using in_array() in the default loose mode.

For example, this currently fails:

Validator::validate(['foo' => true], ['bar' => 'required_if:foo,null']);

// "bar": [
//   "The bar field is required when foo is true."
// ]

This is because the null parameter is converted to a string 'null' and placed in an array, which is passed as the haystack to in_array(), with the dependent field value as the needle. e.g.

in_array(true, ['null']); // true

We can pass true as the third parameter to use strict mode, however many people no doubt rely on it loosely comparing numeric strings and integers, so this PR instead only enables strict mode when the value is a boolean, which fixes the bug hopefully without breaking any expected behavior.

I'm not sure if this is considered a breaking change - I guess that depends on whether anyone is depending on the existing behavior. It does feel risky to tweak the behavior of such fundamental validation rules though.

[driesvints]

@jessarcher is this also broken for v6 LTS? Otherwise it's best that you send this in to 6.x

[jessarcher]

@jessarcher is this also broken for v6 LTS? Otherwise it's best that you send this in to 6.x

Good call @driesvints! I've cherry-picked the commits into a new branch and PR at #36504