MdeModulePkg/UefiHiiLib:Fix incorrect comparison expression

MdeModulePkg/UefiHiiLib:Fix incorrect comparison expression

Fix the incorrect comparison between pointer and constant zero character.

https://bugzilla.tianocore.org/show_bug.cgi?id=416

V2: The pointer StringPtr points to a string returned
by ExtractConfig/ExportConfig, if it is NULL, function
InternalHiiIfrValueAction will return FALSE. So in
current usage model, the StringPtr can not be NULL before
using it, so we can add ASSERT here.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>