Don't try to launch-adjacent to empty task

If the "source task" is empty when calculating launch root,
it means that the new task is "replacing" the source-task
and thus can't launch "adjacent" to it.

Bug: 352193921
Flag: EXEMPT bugfix
Test: open photos link from gmail on large-screen
Change-Id: Iaca491a20f419b75e54e6315477c540272ec63de