StandaloneMmPkg: Fix ECC error 3002 in StandaloneMmCpu

StandaloneMmPkg: Fix ECC error 3002 in StandaloneMmCpu

Bugzilla: 3150 (https://bugzilla.tianocore.org/show_bug.cgi?id=3150)

Fix the ECC tool reported error "[3002] Non-Boolean comparisons
should use a compare operator".

Also fix the following:
 - add curly braces for 'if' condition statements to comply
   with the coding standard.
 - The value returned by GET_GUID_HOB_DATA() is stored in
   *HobData. Therefore, check *HobData against NULL. The
   original code was checking HobData which is incorrect.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>