Do not send empty auth when setting up cross-signing keys (#29914)

* Do not send empty auth when setting up cross-signing keys

My understanding from the spec is that no auth parameter should be sent
when starting a UIA flow. [This section](https://spec.matrix.org/v1.14/client-server-api/#user-interactive-api-in-the-rest-api)
says that "A client should first make a request with no auth parameter"
and this is not what element-web is doing (since it is sending an auth parameter with
an empty dictionary).

In the upload cross-signing keys endpoint
[documentation](https://spec.matrix.org/v1.14/client-server-api/#post_matrixclientv3keysdevice_signingupload_request_authentication-data)
it says that type may be omitted if session is set, but in this specific
case neither of the fields is set.

* fixup! Do not send empty auth when setting up cross-signing keys

test/CreateCrossSigning-test.ts

@@ -45,7 +45,7 @@ describe("CreateCrossSigning", () => {
 
         const makeRequest = jest.fn();
         await authUploadDeviceSigningKeys!(makeRequest);
-        expect(makeRequest).toHaveBeenCalledWith({});
+        expect(makeRequest).toHaveBeenCalledWith(null);
     });
 
     it("should prompt user if upload failed with UIA", async () => {