Issue #3616606: Detect the original entity without property_exists

Active updating promises that files are only processed as they are attached, but the skip that enforces it never fired on Drupal 10.3 to 11.1, so files were renamed on every save. The check used property_exists to find the entity's original, and that is always false for a content entity, whose original is reached through a magic accessor rather than a real property. This takes the original entity object and tests it directly, which works on every supported version. dshields reported it and wrote the fix; the kernel tests are mine, with a control so a passing skip cannot mean the hook simply never ran.