Bug: Transformer does not allow setting attribute/property to `false`

[Menrath]

It should be possible to set an attribute to false. Within the Base::transform_object_properties() function this does not work anymore:

wordpress-activitypub/includes/transformer/class-base.php

Line 93 in 1dbcc8e

if ( ! empty( $value ) ) {

Introduced in #1301

Updates Base::transform_object_properties() and Base_Object::to_array() to not only discard null values but also empty values unless they're false.

Seems like empty( false ) evaluates to true, which I also did not know.