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

[jessarcher]

Copied from #36498 to target 6.x

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.

[Shkeats]

@jessarcher We happened to have some overzealous unit tests covering part of our validation around required_if between two boolean fields and found this change caused them to fail. On the framework 8.x branch. The fix was to change our validation rule configuration from using 0 and 1 to true and false on the required_if rules. Screenshots of the original test and validation rules below.

It seemed to have no impact on the actual form behaviour from the frontend but hopefully might help someone else understand what's going on if they see a similar issue.