Add missing nothingToDraw() check

Fixes: 34598346

Prior to 2874daa4 we
never freed the resources of root nodes. Now that this is being
done we need to ensure that the backdrop has content before
we try to draw that content. All other nodes passed in
deferRenderNodeScene check that nothingToDraw() returns false
before drawing them, but that check was missed on the backdrop
node.

Test: manual. open calculator then long-press on recents
Change-Id: I495749161b38b57278bdb60d5f872eeeb2b218d0