Ignore request with no valid SmartSpaceTargets.

When we receive an empty list of SmartSpaceTargets, it will cause that `rootViewAttached` to be `false`. This will cause the `AmbientCue` to be detached.

This can happen in some cases that other smartspace card requests updates from AiAi, which will occasionally clear the cue bar.

Bug: 425831320
Flag: com.android.systemui.enable_underlay
Test: atest AmbientCueRepositoryTest
Change-Id: Ib3fe868e9767c9332c0134f6c687df3b5f46e5fe