surfaceflinger: skip composition for empty frames

By not committing the results of composition for empty frames, we
avoid spitting out series of black frames for virtual displays that
don't have visible layers. We still draw one black frame when going
from having layers to not having any. In particular, this avoids
having a series of empty frames due to re-compositing the primary
display in the period between creating the virtual display and adding
layers to it.

Bug: 16786752
Change-Id: I7e9b2ed2e407d8d49c7af736b447d4c6181b0ad8