message: Fix vk_imgui crash by allowing zero-sized uniform buffers.
  The vk_imgui sample regressed in December or so, after some changes in
  how Filament manages uniform buffers. This commit makes the Vulkan
  driver more consistent with the GL driver, which does a check for
  zero-sized buffers.
diff: @@ -557,10 +557,12 @@ void VulkanDriver::setExternalStream(Driver::TextureHandle th, Driver::StreamHan
  }
  void VulkanDriver::updateUniformBuffer(Driver::UniformBufferHandle ubh, BufferDescriptor&& data) {
  + if (data.size > 0) {
  auto* buffer = handle_cast<VulkanUniformBuffer>(mHandleMap, ubh);
  buffer->loadFromCpu(data.buffer, (uint32_t) data.size);
  scheduleDestroy(std::move(data));
  }
  +}
  void VulkanDriver::updateSamplerBuffer(Driver::SamplerBufferHandle sbh,
  SamplerBuffer&& samplerBuffer) {
  