Add null check before playing recents scale down anim

When swiping up to overview from an app, we scale down RecentsView in
launcher, synchronized with the app window. In rare cases, however,
there may not be any TaskViews in launcher, so we should just ignore
the launcher component in this case. For example, this happens in
LockTask mode.

Bug: 110703110
Change-Id: Ib6f836581a3e12babf4e26e272b77bb529f86fd4