Demo Mode Signed In: Return early if DM Token or Client ID is empty

The request validator in AuthenticateDmTokenService throws an
exception if either of the DM Token or the Client ID is empty, so an
early return in the client in this case would make sense.

This helps save sending unnecessary requests and save time waiting for
the server to respond with an error.

Additionally, kCannotObtainDMTokenAndClientID is renamed to
kCloudPolicyNotConnected to avoid enum names too similar to each other
to cause a confusion.

TEST=unit tests DemoLoginControllerTest.*

Bug: 390239730
Change-Id: Ic08e9d8b346c8b9e70bace3057e96d5ec1092a62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6266265
Auto-Submit: Haifan Wang <wanghaifan@google.com>
Reviewed-by: Xiqi Ruan <xiqiruan@chromium.org>
Commit-Queue: Haifan Wang <wanghaifan@google.com>
Cr-Commit-Position: refs/heads/main@{#1423433}