Add smartspace complication even when targets empty.

This CL changes the logic to add smartspace complication even when
smartspace targets are empty. The default smartspace card (with
only date) should still be displayed, so to be consistent with the
lockscreen and laucher.

Bug: 243105222
Fix: 243105222
Test: atest SmartSpaceComplicationTest
Test: wipe device, enable art gallery screensaver, see the default
smartspace card shown with only date info

Change-Id: I46af1c5ec017e4eb78ef9cd7e96e887fcb5597bb