Fix EZP-22958: invalid values in subtree criterion handler

JIRA: https://jira.ez.no/browse/EZP-22958

Add proper validation of subtree format (`/1/2/3/`) for all elements in the array $value passed to the Subtree criterion.

Note that this does force the pathString to end with a forward-slash (`/`).
Can't recall the reason for this change, but it seems `$value` is (and was already) accepting both types.
I guess this be type-checked somewhere?
Also
```
@param string|string[] $value an array of subtree path strings, eg: /1/2/ 
```