fix: stop requesting a project by an undefined id after the last one is deleted (#14661)

* fix(frontend): stop querying a project the account no longer has

The existence guard in useGetFolderQuery only ran when `params.id` was
truthy, so an account with no project left fell straight through to the
request and sent the literal string "undefined" as the path segment:
GET /api/v1/projects/undefined -> 422 uuid_parsing.

Both guards now run before the URL is built, and they resolve the query
with `null` instead of returning nothing. Returning `undefined` from a
query function is rejected by react-query, which turned every guarded
read into a failed query plus a dev-only console error; `null` settles
the query as a success carrying no folder, which is what the caller
already reads through optional chaining.

* refactor(frontend): drop the non-null assertion on the project id

`myCollectionId` is `string | null | undefined` and is genuinely empty
once the account holds no project, so asserting it away hid from the
type checker the very state that produced the invalid request. The
folder query now accepts a missing id, so the call site can pass it as
it is.

* test(frontend): cover the project ids useGetFolderQuery must not request

Locks both guards: a missing id (undefined, null or empty) and an id the
folder store no longer lists resolve with null and issue no request,
while an id the store still lists is requested with the id in the path.

Against the previous implementation four of the five cases fail — the
missing-id ones because the request goes out as /projects/undefined, and
the deleted-project one because the guard resolved with undefined.

* test(frontend): fail folder-deletion tests on a non-UUID project request

The suite already walked the one-project-to-none transition that produced
GET /api/v1/projects/undefined, but nothing looked at what the browser
asked for, so the 422 passed unnoticed. Every project request made by
these tests is now checked, and any single-segment project id that is not
a UUID fails the test that issued it.