Fix QuickContact is finished when cancelling link action

If no contact to be linked selected, null intent is returned to
QuickContact Activity. Should just return instead of processing the
intent.

Bug: 32024874

Test: Manually tested when opening QuickContact from main contacts
list, label view, and account view, following repro steps previous
QuickContact view shows.

Change-Id: Id89413ade3f7dac227c160ae5ab8e96bc1e48960