[5.7] Allow to disable CREATED_AT in Models

[TBlindaruk]

Problem

We can disable the UPDATED_AT timestamp in Eloquent (just set the const CREATET_AT to null).

class UserWithCreated extends Eloquent
{
    public const UPDATED_AT = null;

    protected $table = 'users_created_at';
}

After this disabling CREATED_AT will work as expected.

But we cannot disable just CREATED_AT constant.
We got the error

Illuminate\Tests\Database\DatabaseEloquentTimestamps::testUserWithUpdatedAt
ArgumentCountError: Too few arguments to function Illuminate\Database\Eloquent\Model::setAttribute(), 1 passed in
/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php on line 548 and exactly 2 expected

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This PR is allowed to set CREATED_AT to null;

It was already proposed and rejected in
#22990
and
#23638

I have created a Unit test as @tillkruss asked.

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