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 <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>
Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <1e8692d8ed8470b17e8ddd7612a0f029768dcb56@intel.com>
Reviewed-by: Eric Dong <fa9eeec52367040ac98013bc4b2913db170588f8@intel.com>

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