Arrays::get is not type strict

[mirfilip]

There are two problems when using Arrays::get($data, $path, $default);

Consider such input:

$data = [
    'foo' => null,
];

1. When $key is a dot notation for a path that doesn't exist, $default is returned. By default, it'd be null. One can work around this problem as $default can be a Closure. Sth like Arrays::get($data, $path, function() { throw new \Exception('Item not found'); }); works but is just a workaround for another problem. Example:
   Arrays::get($data, 'foo.bar.baz'); returns null.
2. When $key exists but the value is null, it would be treated as not existing. Example: Arrays::get($data, 'foo', 'the value does not exist'); returns the value does not exist

Both problems stem from the fact of using isset() instead of array_key_exists(). It's trivial to fix the issue but it'd introduce BC breaks.