Digital Credentials: Missing empty-vector guard after request validation allows empty picker presentation

https://bugs.webkit.org/show_bug.cgi?id=314382
rdar://176536555

Reviewed by Anne van Kesteren.

Per the Digital Credentials spec's "prepare credential requests" algorithm, if the
validated requests list is empty (all requests filtered out during validation),
reject with a TypeError before presenting the picker. Implement this guard in
CredentialRequestCoordinator::prepareCredentialRequests, immediately after the
validation step, matching the spec rather than guarding at the WebProcess boundary.

Test: http/tests/digital-credentials/digital-credential-all-invalid-requests.https.html

* LayoutTests/http/tests/digital-credentials/digital-credential-all-invalid-requests.https-expected.txt: Added.
* LayoutTests/http/tests/digital-credentials/digital-credential-all-invalid-requests.https.html: Added.
* Source/WebCore/Modules/identity/CredentialRequestCoordinator.cpp:
(WebCore::CredentialRequestCoordinator::prepareCredentialRequests):
* LayoutTests/http/wpt/identity/formats/ISO18013/mixed-requests.https-expected.txt:
* LayoutTests/http/wpt/identity/formats/ISO18013/mixed-requests.https.html:

Canonical link: https://commits.webkit.org/317276@main