text: Title: Consider adding checks to make sure the "filtering" layer's children is not empty.
  Body: Consider adding checks to make sure the "filtering" layer's children is not empty. just like what we did in opacity layer.
  
  * ImageFilterLayer
  * ColorFilterLayer
  * ShaderMaskLayer
  
  c.f.
  https://github.com/flutter/engine/blob/e444009bf46b39af0b2a2aa6e7c28c668c50004c/flow/layers/opacity_layer.cc#L40
  
  