PcAtChipsetPkg/PcRtc: Handle NULL table entry in RSDT/XSDT

The ACPI code may reserve the first entry for a certain table
(might be FACS) to help with OS compatible issues.
We need to skip the NULL table entry in RSDT/XSDT.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>