Fix crash when docking from recents.

The crash was happening because we were sending a future from sysui, but
that future would always return null animation specs when docking. This
makes it returns proper specs in that case. However, we need to still
protect ourselves from null specs when executing the future.

Bug: 25765339
Change-Id: I02d842ea31503169f6e053a1695d8896d2e4d97c