[6.x] Fix updateChildPageUris empty check never short-circuiting

[duncanmcclean]

This pull request fixes an issue where saving a structured entry would rewrite URIs for every entry in the collection, even when the entry has no child pages.

This was happening because empty() was being used to check if the flattenedPages collection was empty. Since empty() on a Collection object always returns false (it's a non-null object), the early return never triggered. This meant updateEntryUris was called with an empty Collection, and in the eloquent driver, the whereIn filter was skipped — causing every entry's URI to be recomputed.

This PR fixes it by using the Collection's isEmpty() method instead of empty().

Fixes #14546