[FormError] Add support for string cause and null cause

[Nek-]

Currently, the FormErrorResponse does not support null and string causes. It may occur more than we think. This should be added.

The following should be enough:

if ($cause === null) {
    return [$formError->getMessage(), $propertyPath];
}

if (is_string($cause)) {
    return [$cause, $propertyPath];
}