message: Fix shape inference for RFFT on empty shapes.
  When the input is a shape with 0 dimensions, the output shape should be the
  same. Otherwise we don't detect that nothing needs to be computed.
diff: @@ -1858,6 +1858,9 @@ ShapeInference::InferDegenerateDimensionBroadcastShape(HloOpcode operation,
  fft_length[i]);
  }
  }
  + if (ShapeUtil::IsZeroElementArray(in)) {
  + return in;
  + }
  Shape result = ShapeUtil::ChangeElementType(in, C64);
  result.set_dimensions(result.dimensions_size() - 1,
  fft_length[fft_rank - 1] / 2 + 1);
  