Optimize the code to not call StrToBuf when DataLength is 0.

Signed-off-by: Ruiyu Ni <ruyu.ni@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15143 6f19259b-4bc3-4df7-8a09-765794883524