Fix NPE when checking if recents should be launched in docked mode.

In some cases the activity starting method might return early with
positive return code, even though it skipped doing most of the work
and the target stack is not set. We need to protect ourselves from
an NPE in that case.

Bug: 26541093

Change-Id: I7227ad8efa30c52dd8178ad3c9d8b2263c89a74f