FIX: Fixes #8459 Missing conditionals for optional constructor args

[phptek]

Fixes #8459

• Missing conditionals for optional constructor args
• Missing calls to FieldList::setForm()
• Missing guards around naive calls to Form::Fields()->foo()

[maxime-rainville]

I think it would be better if the null checks were moved to the setFields() and setActions(). The constructor could just call the setters. That way we avoid duplicating logic between the setters and the constructor.

[kinglozzer]

Would it be easier, rather than scatter around extra null conditional checks, to just add something to the constructor like:

$fields = $fields ? $fields : FieldList::create();
$actions = $actions ? $actions : FieldList::create();

[maxime-rainville]

@kinglozzer's suggestion makes more sense.

[phptek]

Fixed. Force-pushed. Please excuse the unrelated changes, I need newlines to be there and typos not to be.

[maxime-rainville]

Looking good. Tested locally.