Fix parsing multidimensional array from multipart

[Vinimaks]

During parsing a multipart array there is a check if the current segment is empty to set a value. Basically, adding a new element to an array.

It works perfectly fine for simple arrays like:

• 'categories[]: 1'
• 'categories[]: 2'

But sometimes the array key is also specified, fx:

• 'categories[0]: 1'
• 'categories[1]: 2'

Or something more complicated as Laravel Nova does:

• 'options[0][fields][id]: 1'
• 'options[0][fields][participants]: 1'

In that case the check:

if (empty($segment = substr($segment, 0, -1))) {
    $pointer = &$pointer[];
} else {
    $pointer = &$pointer[$segment];
}

Will fail as empty considers the first array key ('0') as true

Thus each time a new array element created instead of adding to the existing one, which causes some unexpected result

Report to that issue in the Nova issues repository: laravel/nova-issues#6229