RegionSamplingListener: Treat empty Rect as BAD_VALUE

The Rect will eventually be merged with other listeners to create an
area of the screen to capture. If the area is empty, this will result in
trying to create an empty texture, which will fail and hit a
LOG_ALWAYS_FATAL.

This means that mDescriptors will only contain non-empty Rects; if
mDescriptors is empty, the RegionSamplingThread will not attempt to do a
capture.

Bug: 206415266
Test: make
Change-Id: Ib395a7627feba73bb9e4ca5f38aebd675515e506