MdeModulePkg/Core/Dxe: downgrade "CodeSegmentCount is 0" msg to DEBUG_WARN

UEFI executables that consist of a single read+write+exec PE/COFF section
trigger this message, but such a binary layout isn't actually an error.
The image can be launched alright, only image protection cannot be applied
to it fully.

One example that elicits the message is (some) Linux kernels (with the EFI
stub of course).

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>