[scenic] Fix swapchain issue when no Vulkan validation layers enabled.

We were unnecessarily querying the number of layer
properties and failing if the number was 0.

SCN-1008 #done

TEST=confirm works on arm64 and x64 devices with
--args=scenic_enable_vulkan_validation=false

Change-Id: I600957ea33ebd3b1be78c52d13f5910be63f09fa
1 file changed