EdkCompatibilityPkg/PrintLite: Fix ErrorPrint() wrong NULL char check

- '\0' -> NULL
- https://bugzilla.tianocore.org/show_bug.cgi?id=47

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Bell Song <binx.song@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

EdkCompatibilityPkg/Foundation/Library/Dxe/PrintLite/StdErr.c

@@ -101,7 +101,7 @@ Returns:
     return 0;
   }
 
-  if (ErrorString != '\0') {
+  if (ErrorString != NULL) {
     if (gST->StdErr != NULL) {
       //
       // To be extra safe make sure StdErr has been initialized