MdeModulePkg AcpiTableDxe: Use Rsdt to check against NULL

MdeModulePkg AcpiTableDxe: Use Rsdt to check against NULL

Some static scan tool may regard CurrentRsdtEntry to be potentially
referenced to NULL pointer if CurrentRsdtEntry == NULL is used in
the right above if condition judgment.

        CopyMem (CurrentRsdtEntry, CurrentRsdtEntry + 1, (*NumberOfTableEntries - Index) * sizeof (UINT32));

It is introduced by commit f9bbb8d9c3f065faba9f266cf4e731fe2ca70c4d.
To avoid it and have same style with
"((Xsdt == NULL) || CurrentTablePointer64 == (UINT64) (UINTN) Table->Table)",
use Rsdt instead of CurrentRsdtEntry to check against NULL.

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