RedfishPkg: add explicit variable initialization

Ancient GCC 4.8.5 warned about variable may be unitialied.
And it doesn't look like false alarm.

The warning is:
edk2/RedfishPkg/Library/HiiUtilityLib/HiiUtilityInternal.c: In function 'GetQuestionDefault':
edk2/RedfishPkg/Library/HiiUtilityLib/HiiUtilityInternal.c:5519:6: error: 'ConfigAccess' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if (ConfigAccess != NULL) {

Cc: Abner Chang <abner.chang@amd.com>
Cc: Igor Kulchytskyy <igork@ami.com>
Reviewed-by: Nickle Wang <nicklew@nvidia.com>

Signed-off-by: Mike Maslenkin <mike.maslenkin@gmail.com>

RedfishPkg/Library/HiiUtilityLib/HiiUtilityInternal.c

@@ -5451,6 +5451,7 @@ GetQuestionDefault (
 
   Status            = EFI_NOT_FOUND;
   StrValue          = NULL;
+  ConfigAccess      = NULL;
   OriginalDefaultId = DefaultId;
   DefaultLink       = GetFirstNode (&FormSet->DefaultStoreListHead);