MdeModulePkg SmiHandlerProfile: Fix no PDB case handling incorrectly

MdeModulePkg SmiHandlerProfile: Fix no PDB case handling incorrectly

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=569

The PdbStringOffset should be set to 0 for no PDB case,
then SmiHandlerProfileInfo can use it to know whether
there is PCD info or not.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>