Do not generate an empty PublicKeyCredentialUserEntity in response to GetAssertion

src/ctap/mod.rs

@@ -762,7 +762,7 @@ where
             key_id: credential.credential_id.clone(),
             transports: None, // You can set USB as a hint here.
         };
-        let user = if flags & UV_FLAG != 0 {
+        let user = if (flags & UV_FLAG != 0) && (credential.user_handle.len() > 0) {
             Some(PublicKeyCredentialUserEntity {
                 user_id: credential.user_handle.clone(),
                 user_name: None,