Fix bug that the dream is set to "" when null is passed.

Currently if null is passed, the active dream is set to "" instead of
null. This is problematic because our code doesn't have logic for
handling "" like null.

Test: atest NudgeToSetupDreamCallbackTest
Test: manually flash device and run
Test: atest DreamManagerServiceTests
Bug: 213368707
Change-Id: Ic4922fb61f7ceef9fc48e81f1455872861e34af5