[4.x] Fix URI validation error when collection route uses entry IDs

[duncanmcclean]

This pull request fixes an issue when creating a new entry, where the collection's route uses the entry ID, like this: /members/{id} or simply /{id}.

Previously, when creating an entry, no ID would exist so the URI would be simplified to /members so if you had an existing entry at /members, it would cause a validation error.

This PR attempts to fix this by always passing an ID into the UrlBuilder if the entry doesn't already have one set.

Fixes #5500.

[jasonvarga]

I think this might break Eloquent since then it will try having a UUID as the ID when it should really generate an ID when it's inserted.

Maybe you can tweak validateUniqueUri to check for $entry->id() being null?