Don't flatten nullptrs

Bug: 28428970

BufferQueueProducer can return a nullptr for
the GraphicBuffer if there's nothing queued, which
IGraphicBufferProducer would attempt to flatten.

It's useful to distinguish that there's nothing queued
instead of just generic error, so fix IGraphicBufferProducer
to handle null GraphicBuffers.

Change-Id: I9e2b11e107e093d209a89a01d14feec3e885f268