D10-9: Safer PHP array destructuring.

[adam-vessey]

Exposed with the bump to PHP 8, started throwing warnings due to attempting to use things naively.

https://www.php.net/manual/en/language.types.array.php :

Attempting to access an array key which has not been defined is the same as accessing any other undefined variable: an E_WARNING-level error message (E_NOTICE-level prior to PHP 8.0.0) will be issued, and the result will be null.

Started popping from our migration code, as we would:

• have a uri such as foxml://object/some:pid
• check that it is not writable
• naively chop to foxml://object, to attempt to assert that the parent is also not writable

Thinking: ObjectLowLevel adapters should be expected to avoid providing files which would be deletable. The akubra_adapter implementation does so... adding in some filters so the "archival" adapter should also.