Fix group summary suppression logic when without bubbles

The current logic of updateSuppression() concludes there is no child
notification when there is no Bubbles defined.

Bug: 161980186
Bug: 174623537
Test: atest SystemUITests:com.android.systemui.statusbar.notification
Change-Id: Iad4ea8117ef9bd7f813854eb6ed353df64fbdedd